| Magnetic impedance of magneto-sensitive materials, under the excitation of an alternating current(AC) with a certain frequency and amplitude, largely varies with the change in the external magnetic fields, which is called as giant magneto-impedance(GMI) effect. As a kind of excellent soft magnetic functional materials, the glass-covered amorphous micro-wires exhibit great potentials in magneto-dependent sensor due to their high sensitivity and quick response. Meanwhile, these wires often offer high resistivity, high magnetic conductivity, excellent electromagnetic matching at microwave band, high surface chemical inertia and tunable stress state, which enable them promising as microwave absorber. In this thesis, glass-covered amorphous micro-wires were selected as modeling materials. Their GMI effects and electromagnetic performances have been systematically studied, including the size effect, temperature response, the modification of GMI effects, and the electromagnetic parameter optimization, wave absorption properties for wire-paraffin composites.The GMI effects of as-prepared Co-based amorphous wires with different sizes were studied under different parameters. The results indicate that the influence of the excitation current on the magnetization and GMI effects is closely related with the applied frequencies. At low frequencies, the increase in the excitation current will result in the decrease in the circular permeability, thus reducing the GMI effects. When the frequencies reach up to >1 MHz, the skin effect, instead of the excitation current, will dominatethe magnetization process. There exists an optimal characteristic current(Ip) at which the wires show the optimum GMI effect at specific frequency, and Ip increases with the increase of AC frequencies. The end-effect and direct-current(DC) resistance are the two main factors for the wire length effect on GMI ratios. The diameter effect on the GMI effect is mainly contributed to the difference inthe structure ordering and internal stress distribution resulting from the various quenching rate. The wires show optimum GMI effects at different frequencies with a length of 20 mm and a diameter of 30 μm.The interfacial stress between the covered glass and metallic core were also calculated. The results indicated that the interfacial stress is closely related with the core ratio(η) of the micro-wires and increases with η value. The relationship between the enhancement of GMI and η of the micro-wires after removing the coated glass was established. The GMI increased from 7.23% to 28.4% at 10 MHz when η increased from 2.39 to 3.53. This suggested that the interfacial stress increased with η, consistent with the calculated results.The temperature dependence of GMI effects of thewires with three states were investigated. The results show that the relationship between temperature and GMI effect related to the frequency of drive current, the DC magnetic field and the state of micro-wires. The GMI effectss gradually decreased for both as-prepared wire and micro-wires after removing the glass cover during the continuous heating process. The annealed wires increased firstly and then decreased, or monotonously increased at different frequencies. The as-prepared micro-wires exhibited better temperature stability than DC annealed wires and micro-wires after removing the glass cover. The temperature stability of all the three wires improved with the increases in both the applied frequency and applied DC magnetic field. Additionally, the temperature response of GMI effects for as-prepared wiresdepend on the heating/cooling process. The GMI effects during cooling process did not show an identical trend with those during heating process, but showed a trend of first decrease and then increase. The peak value of GMI effects after cooling down to room temperature is slightly lower than the initial room temperature value. When applied t wo heating/cooling cycle processes, the temperature stability of GMI effects for the second cycle is better than that for the first cycle. For this case, the GMI effects exhibit the same trend during the second heating/cooling cycle.The GMI effects were optimized using two kinds of Joule annealing processes. The maximum GMI ratio of 223% and the maximum responding sensitivity of 316%/Oe can be achieved by pulse current annealing with optimum process parameters of 140 m A, 480 s and 50 Hz. The instantaneous circular magnetic field induced by pulse current annealing is greater than that induced by DC annealing, facilating the formation of short-range order micro-domains in the wires, improving the spin ordering, and thus leading to better GMI effects than DC annealing. The maximum GMI ratio of 429% and the maximum responding sensitivity of 577 %/Oe can be obtained by cryogenic annealing with optimum process parameters of 280 m A and 300 s. The circular magnetic field can be significantly enhanced under high current. The core of the wire crystallized due to the high current whereas the surface remained its amorphous structure due to liquid nitrogen condition, which causes the decrease in the resistivity and thus the increase in the GMI ratios. Based on it, the stress induced by the large temperature difference between the surface and the core can be relieved by the following DC annealing at the ambient condition, which modified the domain structures and further improved GMI effects.The electromagnetic parameters and microwave absorption properties of the glass-covered Fe-based and Co-based short micro-wires composites bonded by paraffin with different packing ratios were studied. The results indicated that the Co-based composite with a packing ratio of 9 wt.% has the largest electromagnetic loss and best microwave absorption properties. The Co-based composites with 3~4 mm thickness have wider absorption band. The corresponding frequencies range 10~16 GHz. The microwave absorption peak values of the composites with 3.2 mm and 3.5 mm thickness are-38.91 d B and-36.34 d B, respectively. The Fe-based composite with a packing ratio of 7 wt.% shows the largest electromagnetic loss and best microwave absorption property. The Fe-based composites with 2~4 mm thickness have wider absorption band. The corresponding frequencies range 12~18 GHz. The microwave absorption peak values of the composites with 3.0 mm and 3.5 mm thickness are-35.94 d B and-39.54 d B, respectively.The joule annealing process with majorized technological parameterwas following principle of impedance match was conducted on the amorphous microwires. The results show that the electroconductivity increasement was restrained while permeability increases and the coercivity decreases with the annealing process, resulting in an increase of electromagnetic parameter matching for wave-absorbed composites. The absorbing bandwidth was enlarged and the absorption peak area increased for the annealed Co-based microwires enabled paraffin composite compared to the as-cast wire. The largest absorption value of the annealed wire enabled composite was-17.2 d B and the corresponding frequency was 14.6 GHz when the characteristic thickness was 2.5 mm. The absorbing bandwidth exceeds 7.2 GHz with an absorption value larger than-10 d B. The peak value was-19.8 d B and-20.4 d B when the characteristic thickness increased to 3.5 mm and 4 mm, the corresponding frequencies was 10.48 GHz and 9.12 GHz, respectively. The absorbing bandwidths of these two samples are 6.8 GHz when the absorption value larger than-10 d B. |
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