# Optical Fiber Current Sensors Based on FBG and Magnetotstrictive Composite Materials
## 3 Optical Fiber Current Sensor Based on Magnetostirctive Composite Materials
### Magnetstrictive Composites
The author prepared the composite material with 70% particle volume fraction, as shown in Figure 15 for the specific process of preparing the composite material: First, epoxy resin and curing agent are mixed according to a certain proportion, then a certain amount of TD particles are added, then the mixture is fully stirred and degassed in vacuum, then the mixture is poured into a mold, and finally the mold is placed in a magnetic field to wait for curing. When the pre-compression stress and the orientation of the magnetic field intensity are the same, the magnetostriction value of composites with vertical orientation magnetic field direction is greater than that of composites with parallel orientation magnetic field direction. At the same time, with the same orientation magnetic field, when the pre-compression stress is from 0 to 9 MPa, the magnetostrictive value of the composite material gradually increases with the increase of pre-compression stress. When the pre-compression stress reaches 9 MPa, the composite material obtains the highest magnetostriction value. [PERSON] et al. [25] studied the effects of TD particle size, particle volume fraction, binder type, and pre-compression stress on magnetostrictive composites. As shown in Figure 16, the fiber grating is embedded in the magnetostrictive composite material. When the particle volume fraction is 10%, 20%, and 30%, the magnetization of the composites with 30% particle volume fraction is the highest. When the binder is epoxy resin and polyurethane resin, respectively, the magnetization of the composite with epoxy resin as binder is the highest. The author classified the particle size into < 50 \(\upmu\)m, 74-150 \(\upmu\)m and > 200 \(\upmu\)m, among which the magnetization of the composites was the highest when the particle size was > 200 \(\upmu\)m. Although the sensitivity of the composite material is lower than that of the monolithic alloy, the sensitivity and linear range of the sensor are increased by 40% and 20%, respectively, by applying 8.6 MPa preloading stress. Figure 15: Preparation process of magnetostrictive composites. [PERSON] et al. [26] studied the effects of magnetic field orientation and rare-earth elements on magnetostrictive composites. The author prepared two samples of the same composite material, one cured without applying orientation magnetic field, the other cured with orientation magnetic field. It is found that the magnetostriction of composites with orientation magnetic field is more than 4 times that of composites without orientation magnetic field. At the same time, it is found that the magnetostriction of the composites doped with rare-earth element Y is obviously higher than that of the composites undoped with rare-earth element Y. [PERSON] et al. [27] studied the effect of titanate coupling agent on magnetostrictive composites. Because the bonding strength between resin and TD particles will affect the properties of magnetostrictive composites, the author treated TD particles with titanate coupling agent to improve the bonding strength. As shown in Figure 17, when the particle volume fraction is the same, the magnetostriction of the magnetostrictive composite material treated with coupling agent is higher than that of the magnetostrictive composite material without coupling agent treatment. However, the saturation magnetostriction of the two magnetostrictive composites is almost the same. At the same time, the author found that the magnetostrictive composite treated by coupling agent has faster response time, which is beneficial to improve the performance of the sensor. [PERSON] et al. [28] studied the optimum orientation magnetic field strength of magnetostrictive composites with different particle volume fractions. Magnetostrictive composites with TD particle volume fraction of 20%, 30%, and 50% were prepared.