Determining the Effects of Increased Volume Fraction of Barium Hexaferrite as a Magnetic Filler in Magneto-Active Elastomers
Open Access
- Author:
- Haussener, Tyler Kress
- Graduate Program:
- Mechanical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 22, 2016
- Committee Members:
- Paris R Vonlockette, Thesis Advisor/Co-Advisor
Zoubeida Ounaies, Committee Member
Karen Ann Thole, Committee Member - Keywords:
- Barium Ferrite
X-ray Diffraction
Magneto-active elastomers
Vibrating Sample Magnetometry
Magneto-rheological elastomers
Magnetic Materials - Abstract:
- Magneto-active elastomers (MAEs) are a subclass of smart materials that have the ability to actuate and alter their mechanical and magnetic response when subjected to an external magnetic field. To fabricate an MAE, a magnetic material is dispersed in an elastomer matrix. Typically a hard magnetic ferrite is used in conjunction with a silicone elastomer. To create a magnetically aligned MAE, the hard magnetic material is mixed in the elastomer solution, then cured in a strong external magnetic field causing particle alignment. Past research has shown that higher magnetic remanence is desirable to increase MAE actuation and produce the most magnetic work. A high degree of alignment of magnetic particles is expected to result in improved remanence. However, in MAEs with soft magnetic particles cured in a field, past research has shown that the degree of alignment, vs. less ordered clustering, varies with particulate volume content. Consequently, it is important to study the degree to which volume content of magnetic material affects physical particle alignment, bulk magnetic properties, and the coupling between the two in MAEs with hard magnetic filler particles. In this work the physical particle alignment of MAEs fabricated using barium hexaferrite (BaM) as the filler material, Sylgard 184 as the matrix material, and with varying particle concentration of 5 - 30% by volume were estimated using data obtained from x-ray diffraction (XRD) and vibrating sample magnetometry (VSM). To date, studies on MAEs have not yet linked the physical assessment of x-ray diffraction techniques to the magnetic assessment of vibrating sample magnetometry to determine the effects of increase magnetic particle volume fraction on magnetization and alignment as this study proposes. The results of the collected experimental data were analytically classified using assumed Gaussian orientation distribution functions of the magnetic domains determined from VSM and a measure of the degree of orientation of physical crystallites based on the March parameter determined from XRD. Results and analysis of the XRD data showed the degree of physical alignment of BaM in MAEs varies with volume content of magnetic material. The trends in the data suggest poled samples of each volume fraction had significant growth in preferred alignment beyond their unpoled counterparts; the lowest degree of preferred alignment occurred in the 15% by volume samples, an intermediate value. Although 15% by volume showed the lowest degree of preferred alignment among poled batches, the calculated alignment parameter for this volume fraction was still at least 196.1% higher than unpoled samples with the same volume content. As volume fraction increased or decreased away from 15% by volume, physical particle alignment, estimated from XRD data, increased with a more significant increase for decreasing volume fractions. The lowest volume fraction tested, 5% by volume, showed the highest degree of preferred alignment with a calculated alignment parameter 125% above the local minimum at 15%. The highest volume fraction tested, 30% by volume, also showed a heightened degree of preferred alignment with the calculated alignment parameter 74.8% above the local minimum at 15%. The ratio of magnetic remanence to magnetic saturation was used to calculate a width of an assumed magnetic orientation distribution function for all poled samples, showing that low volume fractions have a more narrow distribution of magnetic domains. As volume fraction increased, the distribution parameter also increased and approached a constant value. The 5% by volume samples showed the lowest distribution parameter, 40.2% lower than the average determined for higher volume fractions. A smaller distribution parameter indicates magnetic domains form a more narrow distribution about the direction of poling. This distribution parameter was also compared to the normalized remanent magnetization of each volume fraction. The results showed a linear relationship; as the normalized remanent magnetization increased the distribution parameter decreased proportionally. Comparing the calculated physical crystallite orientation parameter to the magnetic orientation parameter showed similar general trends between microscale particle alignment and bulk magnetic properties in an MAE, where the parameters were monotonic with each other but with a nonlinear relationship. Results of normalized remanent magnetization values across volume fractions suggest there is no increased benefit in bulk magnetization per unit magnetic material by adding more BaM in MAEs. In other words, the results of this study show the efficiency, or the ratio of normalized remanent magnetization to normalized saturation magnetization of a sample, of MAE performance using BaM as a filler material cannot be increased by altering the volume content of the magnetic filler. Although the degree of particle alignment of BaM particles may increase for known volume fractions, the remanent magnetization per unit volume of magnetic material, which is directly related to the work produced by an MAE, remains fairly constant for each volume fraction tested. This indicates the work produced per volume of magnetic material of an MAE cannot be increased by changing the volume content of filler in the material. All volume fractions tested (5%-30% by volume) will produce the same net work per unit volume fraction. Therefore, as MAE applications are designed and scaled larger or smaller the volume content of magnetic filler must also be scaled accordingly. This study is a necessary step toward increasing the efficiency and optimizing the performance of future MAEs as developing means of assessing particle and magnetic alignment relationships.