Insulation treatment of MA ribbon surface

Four types of coating method were studied. The advantages and disadvantages are summarized in Table 2. Only the SiO₂ coating prepared using the sol-gel process satisfied the requirement of low stress and high insulation used for high-performance MA core fabrication. Figure 1(b) shows the sol-gel coating facility and process. The thickness of the wet film was controlled by controlling the spacing of Group 1 rollers. Three small rollers were installed in the oven to prolong the standing time of the ribbon to ensure that the surface of the MA ribbon was completely dried before winding.

Figure 2(a) and (b) show the cross-section observation of the sol-gel/SiO₂ coating on the MA ribbon before and after annealing, respectively, using a Hitachi S-3400N high-resolution scanning electron microscope. Before annealing, the sol-gel film was a coating with an irregular morphology, and the element proportions of Si and O were 50.33% and 37.07%, respectively. A dense SiO₂ inorganic coating was formed with an average thickness of about 2 μm after the annealing at 500-600 ℃ (Fig. 2(b)). The element proportions of Si and O were 50.85% and 35.99%, respectively, which were consistent with the element proportions before annealing, indicating that the sol-gel/SiO₂ coating had good thermal stability. An insulation tester (Fluke 1550C) was used to test the voltage hold-off on the annealed MA ribbon. The selected positions of the MA ribbon samples were located at the inner diameter, middle, and te outer diameter areas of the MA core, as shown by the symbols B1, B2, and B3, respectively, in Fig. 3(a). Testing was conducted along the 2.5 cm width of the MA ribbon. The results of the inner, middle, and outer regions are indicated by the black, red, and blue scatters, respectively, in Fig. 2(c). The symbols a1 and a3 represent the areas 5 mm away from the edge of the MA ribbon, and a2 represents the middle region. The average breakdown voltages of regions a1, a2, and a3 were 214, 253, and 278 Vdc, respectively. Figure 2(d) is a histogram of the frequency statistics of all the results. The test data satisfied the Gaussian distribution, whose median and mean were 247 and 250.3 Vdc, respectively. The MA core used in the CSNS-II/RCS MA loaded cavity has a lamination coefficient of 0.75. According to its definition, the induced RF voltage is about 2 V with the ribbon spacing of 6 μm [15]. The test results indicated that the developed sol-gel/SiO₂ coating can satisfy the RF voltage insulation requirement of CSNS-II/RCS MA core and with a sufficient engineering margin.

Fig. 2Full-screen View

(Color online) Cross-sections of the sol-gel coating on the surface of the MA ribbon before and after annealing (a) and (b); Ultimate breakdown voltage of the samples in the different regions of MA core (c); Frequency statistics of all ultimate breakdown voltage measurements (d).

Fig. 3Full-screen View

(Color online) Appearance of the MA core with the size of O.D.850 mm×I.D.316 mm×H.25 mm after annealing (a); Comparison of the measured and theoretical values of radial DC resistance of the MA core (b).

A thermal shock experiment was performed on the coated MA ribbon to evaluate the possibility of coating cracking in long-term engineering use, as the difference between the thermal expansion coefficients of SiO₂ and MA ribbon was large, which were about 3×10^-6/K and 10.6×10^-6/K, respectively [16]. The coated MA ribbon samples with a length of 5 cm were heated from room temperature to 120 °C within 3 min; the ribbon was maintained at 120 °C for 1 h, and then it was cooled to room temperature within 3 min. This process was repeated 200 times. The results showed that the appearance of the MA ribbon coating was complete without apparent peeling off, and the insulation performance and energy dispersive spectroscopy element ratio of the MA ribbon coating were almost the same as those before the thermal shock test, which further proved the excellent thermal stability and adhesion of the sol-gel/SiO₂ coating.

The effects of coating on the RF performance of the MA core were also evaluated by winding MA cores of different sizes. An SY-8232 type IWATSU B-H analyzer was used to measure the RF loss (P_cv), and the average ${\mu }_{\text{p}}^{\text{'}}Qf$ products were measured using a vector network analyzer. The measurement methods and theoretical derivation of the ${\mu }_{\text{p}}^{\text{'}}Qf$ product are introduced in Ref. [17]. The detail test condition and results are listed in Table 3. The results showed that the average RF loss of the coated MA cores was significantly lower than the un-coated MA cores (CK), and the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the coated-MA core increased by about 30% at 1, 3, and 5 MHz. These results confirmed that the sol-gel/SiO₂ coating has lower stress and can effectively reduce the high-frequency eddy current loss and improve the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the MA core.

Large MA core winding

A horizontal winding process was adopted for a large and high-filling-factor MA core winding instead of the traditional vertical winding process, which causes significant damage to the MA ribbon coating [18]. The weight of the MA core was evenly distributed on the support backplane, and the tension required for the MA core winding was significantly reduced. The ribbon tension was kept as low and constant as possible to ensure that the stress distribution of each layer of the MA core was uniform by dynamically adjusting the rotational speed of the unwinding and rewinding motors. In addition, we developed a multi-core lap joint process. A large MA core was composed of several small coated MA cores as the front end of the last roll was mounted on the back end of the first roll.

Figure 3(a) shows a MA core with a size of O.D.850 mm×I.D.316 mm×H.25 mm after winding. We measured the radial resistance of the MA core using a digital multimeter to evaluate whether the inter-layer insulating coating of the MA core was peeled off after winding [18]. To reduce the measurement error, we measured three path lines on the end face of the MA core at 120° intervals along the core circumferential direction, as shown by the three red dotted lines in Fig. 2(a). The average value of the three lines at each the 25 mm interval points were considered the final radial resistance measurement results. Figure 3(b) shows a comparison of the radial resistance distribution of the two MA cores denoted as “850-1” and “850-2” and their theoretical values, in which “Theoretic value” is the theoretical calculation value of the radial resistance in the assumption that the inter-layer insulation of the MA core is good. We observed that the measured results could be better distributed near the theoretical value, which demonstrated that the horizontal winding process has minimal influence on the coating of the MA ribbon [18]. We also observed that the results could completely coincide with the theoretical value. The main reason is that the thickness and resistivity of the MA ribbon fluctuated typically within the range of ±5% during production, and a measurement error was also introduced.

MA core annealing with a transverse magnetic field

A segmented pre-annealing process was developed to solve the problems of temperature lag and overshoot in the large MA core during the annealing process [19]. This method is beneficial to maintaining a good follow-up between the temperature change of the MA core and the preset temperature of the annealing furnace and gradually releasing the amorphous latent heat. Additionally, a static magnetic field was applied parallel to the axial direction of the center of the MA core during the annealing process to improve the ${\mu }_{\text{p}}^{\text{'}}Qf$ product of the MA core [20].

Figure 4 shows the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the MA core developed in this study and a comparison with the J-PARC MA core [7]. The abscissa is the measured frequency range of 0.5-6 MHz, and the ordinate is the ${\mu }_{\text{p}}^{\text{'}}Qf$ product in GHz. The red scatter line in Fig. 4 is the performance level of the MA core with the size of O.D.854 mm×I.D.375 mm×H.35 mm currently used in J-PARC/RCS. It was prepared using an 18 μm FINEMET MA ribbon with a SiO₂ coating of about 2 μm and without a transverse magnetic field process, denoted as FT-3M (18 μm). The ${\mu }_{\text{p}}^{\text{'}}Qf$ products were about 4, 5.5, and 6.5 GHz at 1, 3, and 5 MHz, respectively. The ${\mu }_{\text{p}}^{\text{'}}Qf$ products were improved to 6.9, 10.2, and 12 GHz at 1, 3, and 5 MHz, respectively, after the ribbon thickness was reduced to 13 μm and a transverse magnetic field was applied during the annealing process [7], indicated as FT-3L (13 μm) by the black scatter line in Fig. 4. Note that the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the developed domestic MA core with the size of O.D.850 mm×I.D.316 mm×H.25 mm prepared using the 18 μm 1k107B MA ribbon was close to FT-3L (13 μm), as shown by the blue solid line in Fig. 4. It is now is in the stage of mass production and will be used in CSNS-II/RCS. As expected, the ${\mu }_{\text{p}}^{\text{'}}Qf$ products can be further improved by using a 13 μm MA ribbon [21]. For the experimental MA core with a size of O.D.250 mm×I.D.100 mm×H.25 mm, the ${\mu }_{\text{p}}^{\text{'}}Qf$ products further increased to 9.2, 13, and 15 GHz at 1, 3, and 5 MHz, respectively, as shown by green solid line in Fig. 4. However, we observed a phenomenon in which when the 13 μm 1K107 MA ribbon was used to prepare a MA core with larger size, only about 80% of the performance of small core could be achieved. The reason has not been determined yet, but it may be explained by the quality instability of 13 μm MA ribbon mass production in domestic factories and the change in the crystallization thermodynamic behavior in the thin MA ribbon. The annealing and magnetization process of thin MA ribbon wound core remains to be further studied, but it will become the optimization objective of domestic higher-performance and large MA cores in the future.

Fig. 4Full-screen View

(Color online) Comparison of the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the developed and J-PARC MA cores

MA core waterproof package

An epoxy curing encapsulation process is often adopted to avoid the long-term erosion to the MA core by water [8]. However, the cured epoxy resin has a large stress, which results in a large decrease in the high-frequency performance of the MA core. Through numerical simulation and experiments, Morita et al. confirmed that the epoxy resin infiltrating an MA core is what generates a strong stress in the circumferential direction under a high temperature [8, 16]. Therefore, we designed a new type of packaging structure that can effectively prevent the epoxy resin from infiltrating the interior of MA core. The packaging structure is described as follows: The inner and outer diameter of the MA core is supported by a fiberglass-reinforced tube with a thickness of about 3 mm. The first layer on the end face of the MA core is a coating containing large particles of SiO₂ with a thickness of about 50 μm to fill the tiny gap between the MA ribbons, preventing the subsequent penetration of epoxy into the interior of MA core. Additionally, it provides a good insulation on the MA core end face. The second layer is a cured layer formed by high viscosity epoxy resin and glass fiber cloth, and its main function is to preliminarily shape the MA core. The final layer is a leveling layer formed by the curing of low-concentration epoxy resin and fiberglass cloth; thus, the MA core has sufficient mechanical strength and is waterproof while smoothing the MA core surface. The low-concentration epoxy wraps the entire MA core during curing, and the inner and outer diameter corners of the MA core will also be sufficiently protected. The entire curing process was completed in a vacuum chamber. After curing, the total thickness of the encapsulation layer was less than 1 mm, the thermal conductivity was about 0.5 W/m/K, and the maximum temperature resistance of the encapsulation layer was about 150 ℃. Figure 5(a) shows the appearance of the MA core after packaging.

Fig. 5Full-screen View

(Color online) Appearance of the MA core after waterproof curing and encapsulation (a). Comparison of the ${\mu }_{\text{p}}^{\text{'}}Qf$ product of MA core in each process of packaging.

Because the MA material is sensitive to the stress, the infiltration of epoxy resin and water into the interior of MA core causes a change in the ${\mu }_{\text{p}}^{\text{'}}Qf$ products [22]. We evaluated whether epoxy resin or water penetrates the MA core by measuring the change in the ${\mu }_{\text{p}}^{\text{'}}Qf$ product of the MA core in each curing process and a waterproof test. The ${\mu }_{\text{p}}^{\text{'}}Qf$ product of the MA core after annealing is shown by the black rectangle solid line in Fig. 5(b). After the first layer of SiO₂ treatment, the maximum decrease ratio of the ${\mu }_{\text{p}}^{\text{'}}Qf$ product in the range of 1-10 MHz was less than 2%. The reason was that the thickness of the SiO₂ coating was about 50 μm, which induced a certain stress on the end face of the MA core, but the effect was small. After the epoxy resin cured and encapsulated, the ${\mu }_{\text{p}}^{\text{'}}Qf$ products had almost no change, which indicated that the epoxy resin had not penetrated the MA core, as shown by the blue upper triangle solid line. After packaging, the MA core was immersed in circulating deionized water at 25 ℃ for 30 days, and the water pressure was maintained at 0.3 MPa (simulating an actual operating environment). The ${\mu }_{\text{p}}^{\text{'}}Qf$ products remained stable, which is indicated by the pink lower triangle solid line in Fig. 5(b). The encapsulation layer on the surface of the MA core was complete and had no apparent color change. The experimental results preliminarily confirmed that the MA core had a good waterproofing ability.

Similar to the MA ribbon coating, a thermal shock experiment was performed as the thermal expansion coefficient of epoxy resin in the encapsulation layer was 60×10^-6/K, which was six times higher than the thermal expansion coefficient of the MA material. There would be a risk of stress fatigue in the encapsulated MA core. The encapsulated MA core was placed in an oven as the temperature varied from 25 to 120 °C within 10 min, maintained for 2 h, and finally cooled to the room temperature within 10 min. This process was repeated 100 times. The experimental results showed that the appearance and the ${\mu }_{\text{p}}^{\text{'}}Qf$ products of the MA core had no apparent changes after the thermal shock test. This proved that the encapsulation layer had a significant hardness and adhesion. The waterproof performance and transient thermal shock resistance of the MA core encapsulation layer will also be evaluated in subsequent high-power tests under actual operating conditions.