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In this study, a differential method using refractive index was evaluated as a technique for measuring the adsorption of polyethyleneimine (PEI) on silicon carbide (SiC) and boron carbide (B4C). The results show that while the additional information collected (zeta-potential, sedimentation and viscosity) provides evidence that the powders have adsorbed PEI, the refractive index change method may not be suitable for measuring PEI adsorption.
INTRODUCTION
BACKGROUND
Colloidal Processing
The most common polytypes are 6H (α), 3C (β), and 9R (α).4 The “C,” “H,” and “R” refer to standard crystallographic categories of cubic, hexagonal, and rhombohedral SiC, respectively. Conversely, when sedimentation is low, the absolute value of the zeta potential is high. Additional plots of the zeta potential as a function of voltage can be found in the appendix.
These were used to generate an equation relating the refractive index of the supernatant to PEI concentration and solution pH. The IEP is between pH 2-4 and the zeta potential is negative at higher pH values. SiC refractive index values are closer to water than B4C refractive index values.
The results show that as a function of wt%, the refractive index increases linearly after one measurement (r2 = 0.994). The temperature of the suspensions is proposed to be one reason why the refractive index increases with successive measurements (the refractive index increases with time). Since the refractometer only measures in C, it appears that temperature is not affecting the refractive index.
The refractive index difference method works perfectly with PEI in solution as a function of PEI concentration and pH.
DLVO Theory
Structure
SiC
It is not widely available (currently) and has only a small number of commercial uses.
B 4 C
Surface Chemistry
This could explain the decrease in viscosity and is also visible in the zeta potential curves. As oxidation occurs, water interacts with the SiO2 surface of SiC according to the following equations:14,15. The maximum adsorption level will occur when the powder is oxidized at 550 °C, because the viscosity is then minimal.
Other articles discuss using XPS, Auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and TEM to analyze the carbon and silica of the SiC surface.10–14,16. Abrasion grinding can also help create new surfaces that do not contain B2O3, which have been demonstrated to improve rheological properties.
Polyethylenime (PEI)
Structure
Adsorption of PEI on SiC and B 4 C
Effect on Zeta-Potential
Refractive Index
EXPERIMENTAL PROCEDURE
Powder Acquisition
Powder Characterization
- Helium Pycnometer Density
- Particle Size Distribution (PSD)
- Zeta-Potential
- Sedimentation
To test this assumption, the shape factor "x" was proposed to be a constant ratio for the four B4C samples (Equation 17) to their spherical SSA equivalent. The shape factors for the three measured samples were constant, but the predicted shape factor for B4 was 13.33, which is much higher than the other three powders. Samples were taken from the diluted suspensions with a glass pipette and dropped onto a glass microscope slide.
Zeta potential was measured using a microelectrophoretic method (Laser Zee Meter, Pen Kem, Inc., Bedford Hills, NY, U.S.A.). Samples were prepared by mixing PEI in solution, adding the powder to the solution and adjusting the pH. The polymer dispersion values of 0.46 mg/m2 PEI31 in SiC and 1.83 mg/m2 PEI17 in B4C were found in the literature and were used in all zeta potential measurements.
The correction factor for temperature was not applied because the laboratory temperature was estimated to be within 298 ± 5 K, which would not significantly affect the zeta-potential results. For all zeta potential data sets with standard deviation bars, the data are assumed to be normally distributed. Sedimentation and packing efficiency were observed by creating suspensions of 0.0 to 2.0 mg/m2 PEI in 0.4 mg/m2 increments to settle over time in graduated cylinders.
Samples were prepared using a loading of 20 vol% solids and diluted to 100 mL after being added to the graduated cylinder. The pH was held constant at 3.0 as the PEI level increased in some samples, and the pH shifted as the PEI level increased in other samples. To accurately determine packing efficiency, the graduated cylinders were weighed and the rule of mixtures was used to determine the exact volume of powder and water for each suspension (the weight was consistent across sets of graduated cylinders containing the same powder).
Rheology
Viscosity
Polyelectrolyte Adsorption
PEI Adsorption
Calibration curves using only PEI in water were developed as functions of PEI concentration and pH. When all the powder was centrifuged from the suspension, the supernatant would have less PEI than the original solution. The equation was used to calculate the amount of PEI remaining in solution and the difference between the original amount of PEI and the amount remaining in solution after centrifugation.
This difference can then be converted from the weight percent PEI (wt%) to mg/m2 if the surface area of the powder is known. The refractive index of the supernatant was measured 10 times in a row without replacing the liquid in the cell to determine whether the refractive index changed with each measurement.
RESULTS AND DISCUSSION
Evidence For Adsorption
Viscosity
Sundlof showed that the viscosity reduction of Al2O3 suspensions is at least three orders of magnitude with several different dispersing agents.57. The magnitude of the Hamaker constant is controlled by dipole interactions between the particle and the medium.1 DeCarlo showed that the Hamaker constant can be estimated from the bandwidth of the material. Figure 28 shows the correlation of the approximate Hamaker constant with the determined Hamaker constant using the adopted UV frequency.
Oxide materials tend to follow the y=x line, showing that the correlation between the band gap and the Cauchy Hamaker constant is high. The viscosity will increase as a function of increasing Hamaker constant due to increasing particle-particle interactions. The viscosity of a suspension will increase as the amount of solids increases and the dispersant level is constant.
The viscosity then begins to increase at a decreasing rate and levels off at 1.0 Pa*s around 2.0 mg/m2. The viscosity of S3 SiC follows a similar trend to B2 B4C in that the viscosity first increases and then decreases. However, the viscosity of S3 is several orders of magnitude lower than that of B2 and approaches the viscosity of water at around 1.2 mg/m2.
The solids loading was 35 vol%, so the viscosity was expected to be higher. The viscosity of B BC is shown as a function of PEI content with pH held at. Correlation of the approximate Hamaker constant with the Hamaker constant using the accepted UV frequency.3.
PEI Adsorption
Analysis of calibration curves
With that value it can be plugged into Equation 21 to give an adsorption value in mg/m2, where SA is the total surface area of the powder in the suspension. An example of adsorption calculation is shown below, with the assumptions that the [PEI] remaining in solution = 0.05 wt% and that the SA = 100 m2. Thus, if 0.05 wt% PEI remains in solution and the total surface area of the powder is 100 m2, the amount of PEI adsorbed is calculated as 0.2 mg/m2.
Analysis of refractive index difference method
From this graph, we can conclude that the refractive index measurements of water and B4C are consistent and do not change with time because the slopes are almost zero. This is shown in Figure 34, where the values from Figure 33 have been averaged and plotted as a function of added powder (wt%). It was found that the refractive index would increase with increasing number of consecutive tests if 10 consecutive measurements were taken for each suspension (without sample replacement).
Since the refractive index of SiC samples increases with cumulative tests, the rule of mixtures can be used to calculate how much more SiC is in suspension after 10 consecutive measurements. The volume fraction of SiC and B4C required to achieve this refractive index value in a solution of only water and powder is 0.017 vol% and 0.012 vol%, respectively. A large amount of powder is not required to increase the refractive index of the suspension, even after the suspension has been centrifuged.
The refractive index increases with increasing successive measurements (when the sample is not changed), and the temperature remains constant. Based on the data presented in this study, the PEI adsorption data for SiC and B4C should show evidence that the difference method using the refractive index does not work for the adsorption of polyelectrolytes in ceramic powders. Each of the six adsorption plots in Figure 40 (SiC) and Figure 41 (B4C) demonstrates that the refractive index difference method is not suitable for measuring the adsorption of polyelectrolytes in ceramic powders.
The PEI remaining in solution for SiC is plotted as a function of surface area added in Fig. 40a and calculated using Equation 20. In Fig. 41a, the amount of PEI remaining in solution increases not only as a function of surface area ( calculated from Eq. 20), also indicates that there is more PEI left in the solution than there was initially. This is a physically impossible circumstance and calls into question the reliability of this use of the refractive index.
SUMMARY AND CONCLUSIONS
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Hawn, “Aqueous Dispersion and Slip Casting of Boron Carbide Powder: Effect of pH and Oxygen Content,” J. Joshi, “Effect of Polyethyleneimine Concentration and Molecular Weight on the Zeta Potential, Isoelectric Point of Nanocrystalline Silicon Carbide, and equanol, ” Ceram. Sakka, "Effect of Polyethyleneimine on the Dispersion and Electrophoretic Deposition of Aqueous Suspensions of Nanosized Titania," J.
Neubrand, "Effects of particle size and molecular weight of polyethyleneimine on properties of nanoparticulate silicon dispersions," J.
