Annealing method of anode foil for aluminum electrolytic capacitor

Abstract

The present disclosure provides an annealing method of an anode foil for aluminum electrolytic capacitor, and relates to the technical field of capacitors. The annealing method of the anode foil for aluminum electrolytic capacitor comprises the following steps: placing a target aluminum roll in an annealing furnace, wherein the target aluminum roll is obtained by at least winding the anode foil for aluminum electrolytic capacitor; controlling the furnace pressure of the annealing furnace in a first furnace pressure range and continuously supplying a first gas into the annealing furnace, and then heating the annealing furnace to a first temperature and keeping the temperature for a first time; stopping the supply of the first gas, and vacuumizing the annealing furnace to make the furnace pressure of the annealing furnace lower than a pressure threshold; controlling the furnace pressure of the annealing furnace in a second furnace pressure range and continuously supplying a second gas into the annealing furnace, and then heating the annealing furnace to a second temperature and keeping the temperature for a second time; and cooling the annealing furnace to a third temperature to complete the annealing. The present disclosure can improve the annealing effect of the anode foil for aluminum electrolytic capacitor, and help to improve the product quality.

Description

Technical Field

[0001] This disclosure relates to the field of capacitor technology, and more specifically, to an annealing method for anode foil used in aluminum electrolytic capacitors. Background Technology

[0002] Capacitors are among the most widely used electronic components in electronic devices, serving applications such as DC blocking and AC switching, coupling, bypassing, filtering, tuning circuits, energy conversion, and control. Aluminum electrolytic capacitors, in particular, are widely used in various electronic devices due to their low cost and high specific capacitance.

[0003] Currently, there is a problem with the annealing process of the anode foil used in aluminum electrolytic capacitors, which results in poor process quality and seriously affects product quality.

[0004] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0005] The purpose of this disclosure is to provide an annealing method for anode foil used in aluminum electrolytic capacitors, thereby overcoming, at least to some extent, the problem of poor annealing effect.

[0006] According to a first aspect of this disclosure, an annealing method for an anode foil for an aluminum electrolytic capacitor is provided, comprising: placing a target aluminum coil in an annealing furnace, the target aluminum coil being obtained by at least winding an anode foil for an aluminum electrolytic capacitor; heating the annealing furnace to a first temperature and holding it at that temperature for a first time while the furnace pressure is controlled within a first furnace pressure range and a first gas is continuously introduced into the annealing furnace; stopping the introduction of the first gas and evacuating the furnace to reduce the furnace pressure to a pressure threshold; heating the annealing furnace to a second temperature and holding it at that temperature for a second time while the furnace pressure is controlled within a second furnace pressure range and a second gas is continuously introduced into the annealing furnace; and cooling the annealing furnace to a third temperature to complete the annealing.

[0007] Optionally, the annealing method further includes: winding the aluminum electrolytic capacitor with an anode foil to obtain a raw aluminum coil; and covering the raw aluminum coil with a protective material to obtain a target aluminum coil.

[0008] Optionally, both the first and second furnace pressure ranges are negative pressure ranges.

[0009] Optionally, the pressure range inside the first furnace is 3000 Pa to 80000 Pa, and the pressure range inside the second furnace is 35000 Pa to 90000 Pa.

[0010] Optionally, the first temperature ranges from 300°C to 550°C, and the first time ranges from 6 hours to 12 hours.

[0011] Optionally, heating the annealing furnace to the first temperature includes: heating the annealing furnace to the first temperature at a first heating rate; wherein the first heating rate ranges from 1°C / min to 6°C / min.

[0012] Optionally, the second temperature ranges from 600°C to 660°C, and the second time ranges from 3 hours to 9 hours.

[0013] Optionally, heating the annealing furnace to the second temperature includes: heating the annealing furnace to the second temperature at a second heating rate; wherein the second heating rate ranges from 1°C / min to 3°C / min.

[0014] Optionally, the third temperature ranges from 60°C to 150°C, and the cooling rate of the annealing furnace to the third temperature ranges from 3°C / min to 8°C / min.

[0015] Optionally, the first gas is air, and the second gas is argon or a reducing protective gas.

[0016] In the exemplary embodiments of this disclosure, a segmented annealing method is adopted, which overcomes the problem of foil quality deterioration caused by inconsistent temperature differences between the inside and outside of the foil due to a single annealing process. This improves the effectiveness of the annealing process and contributes to product quality improvement. In addition, the segmented annealing method can effectively reduce energy consumption, save costs, and is suitable for industrial production.

[0017] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0018] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure. It is obvious that the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0019] Figure 1 A flowchart illustrating the annealing method for the anode foil of an aluminum electrolytic capacitor according to an embodiment of the present disclosure is shown.

[0020] Figure 2 A flowchart illustrating the process of obtaining a target aluminum coil according to some embodiments of the present disclosure is shown.

[0021] Figure 3 A photograph of the annealed surface of Embodiment 1 of this disclosure is shown.

[0022] Figure 4 A photograph of the annealed surface of Embodiment 2 of this disclosure is shown.

[0023] Figure 5 A photograph of the annealed surface of Comparative Example 1 of this disclosure is shown.

[0024] Figure 6 A photograph of the annealed surface of Comparative Example 2 of this disclosure is shown. Detailed Implementation

[0025] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided to make this disclosure more comprehensive and complete, and to fully convey the concept of the exemplary embodiments to those skilled in the art. The described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to give a full understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure can be practiced with one or more of these specific details omitted, or other methods, processes, steps, etc., can be employed. In other instances, well-known technical solutions are not shown or described in detail to avoid obscuring various aspects of this disclosure.

[0026] Furthermore, the accompanying drawings are merely illustrative of this disclosure and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted. The flowcharts shown in the drawings are merely exemplary illustrations and do not necessarily include all steps. For example, some steps may be broken down, while others may be combined or partially combined; therefore, the actual order of execution may change depending on the actual situation. Additionally, all terms such as "first," "second," and "third" below are for distinguishing purposes only and should not be construed as limiting the content of this disclosure.

[0027] Currently, porous anode foils are mainly produced using vacuum annealing and protective atmosphere annealing. However, during the manufacturing process, it has been found that vacuum sintering increases the temperature difference between the inner and outer rings of the foil roll, leading to significant performance changes. Protective atmosphere annealing, on the other hand, leaves substantial organic residues on both ends of the aluminum roll, and the adhesion between the edge coating and the core substrate is weak. Overall, the current annealing process is ineffective, significantly reducing product performance and appearance consistency.

[0028] In view of this, the present disclosure provides a new annealing method for anode foil for aluminum electrolytic capacitors, so as to improve the annealing quality to at least a certain extent.

[0029] Figure 1A flowchart illustrating an annealing method for anode foil of an aluminum electrolytic capacitor according to an embodiment of the present disclosure is shown schematically. (Reference) Figure 1 The annealing method for the anode foil of the aluminum electrolytic capacitor may include the following steps: S10. The target aluminum coil is placed in an annealing furnace and is obtained by at least winding the aluminum electrolytic capacitor with anode foil.

[0030] According to some embodiments of this disclosure, the target aluminum coil is an aluminum coil obtained by winding an anode foil from an aluminum electrolytic capacitor.

[0031] The process of preparing anode foil for aluminum electrolytic capacitors is illustrated below.

[0032] First, aluminum powder, binder, curing agent, and organic solvent within a certain particle size range (e.g., 1μm to 10μm) are mixed to obtain a viscous slurry, which can be simply referred to as slurry or aluminum paste. The viscous slurry comprises, by mass percentage, 50wt% to 70wt% aluminum powder, 1wt% to 10wt% binder, and 25wt% to 45wt% organic solvent.

[0033] Specifically, the binder can be one or more of polyvinylidene fluoride (PVDF), polyacrylamide, polyaniline, methylcellulose, and ethylcellulose. The resin binder can completely volatilize during the sintering process and will not remain in the annealed foil.

[0034] Organic solvents can be, for example, ethanol, toluene, acetone, lipid organic solvents, etc. This disclosure does not limit them.

[0035] Next, the viscous slurry can be coated onto the aluminum foil substrate to obtain aluminum foil after the coating process, and the aluminum coil obtained after winding is used as the target aluminum coil.

[0036] In addition, it was found during the process that the circulating hot air in the annealing furnace directly acts on the surface of the foil during the annealing process, which causes the aluminum powder coating on the surface and ends of the foil to fall off, affecting the quality of the anode foil.

[0037] To address this issue, in some embodiments of this disclosure, the aluminum coil obtained after winding can be referred to as the original aluminum coil. Before entering the furnace, the original aluminum coil is coated with a protective material to obtain the target aluminum coil. This disclosure does not limit the protective material; for example, it can be aluminum foil, and the thickness of the aluminum foil can be different from the thickness of the aluminum foil substrate. It is understood that the above-mentioned coating operation refers to coating the entire original aluminum coil to prevent aluminum powder from falling off the foil. Alternatively, the above-mentioned coating may refer to coating only the areas or portions of the aluminum coil that may come into contact with the circulating hot air.

[0038] For details, please refer to Figure 2Steps S20 to S26 in the present invention describe the process of obtaining the target aluminum coil in these embodiments.

[0039] In step S20, aluminum powder, binder, curing agent and organic solvent can be mixed to obtain a viscous slurry. The quality is as described above and will not be repeated.

[0040] In step S22, the viscous slurry can be coated onto the aluminum foil substrate to obtain aluminum foil with the coating process completed, and then wound into an original aluminum roll.

[0041] In step S24, the raw aluminum coil can be placed in the annealing area for 24 to 48 hours. The annealing area is a material preparation area used for temporary storage of aluminum coils during production or experimentation.

[0042] In step S26, three to five layers of aluminum foil with a thickness of 0.06 mm to 0.2 mm can be wrapped around the outer periphery and ends of the original aluminum coil, and the gaps are sealed with aluminum tape to obtain the target aluminum coil.

[0043] After obtaining the target aluminum coil, it can be placed in an annealing furnace, and the furnace circulation fan and negative pressure vacuum pump can be turned on.

[0044] S12. With the furnace pressure controlled within the first furnace pressure range and the first gas continuously introduced into the annealing furnace, the annealing furnace is heated to the first temperature and held at that temperature for the first time.

[0045] In an exemplary embodiment of this disclosure, the first furnace pressure range is a negative pressure range. For example, the first furnace pressure range can be from 3000 Pa to 80000 Pa. The first temperature range is from 300°C to 550°C, and the first time range is from 6 h to 12 h.

[0046] Preferably, the first furnace pressure range can be controlled between 30,000 Pa and 50,000 Pa, the first temperature range can be controlled between 350°C and 500°C, and the first time range can be controlled between 8 hours and 10 hours.

[0047] Specifically, the annealing furnace can be heated to a first temperature at a first heating rate. For example, the range of the first heating rate can be from 1°C / min to 6°C / min.

[0048] According to some embodiments of this disclosure, the first gas is air. Continuously introducing air allows it to react with residual solvents, binders, and other organic matter inside the coating. Under negative pressure, volatile organic compounds and their reaction products with air are promptly discharged, and the continuous airflow carries away these waste gases, preventing them from accumulating on the outer edge of the aluminum coil end face and forming brown or black patches that would affect the sintering bond between the aluminum powder and the foil in the next step.

[0049] S14. Stop introducing the first gas and evacuate the furnace to make the pressure inside the annealing furnace lower than the pressure threshold.

[0050] After heating the annealing furnace to the first temperature and holding it at that temperature for the first time in step S12, the introduction of the first gas can be stopped, and the annealing furnace can be evacuated to reduce the pressure inside the furnace to below the pressure threshold. The pressure threshold mentioned in this disclosure can be, for example, 200 Pa, 230 Pa, 300 Pa, etc., and this disclosure does not limit it.

[0051] S16. With the furnace pressure controlled within the second furnace pressure range and the second gas continuously introduced, the annealing furnace is heated to the second temperature and held at that temperature for the second time.

[0052] After step S14, another stage of the annealing process of this disclosure can be performed.

[0053] In an exemplary embodiment of this disclosure, the pressure range inside the second furnace is also a negative pressure range. For example, the pressure range inside the second furnace can be from 35,000 Pa to 90,000 Pa. The second temperature range is from 600°C to 660°C, and the second time range is from 3 hours to 9 hours.

[0054] Preferably, the pressure inside the second furnace can be controlled between 65,000 Pa and 75,000 Pa, the temperature can be controlled between 630°C and 660°C, and the time can be controlled between 7 hours and 9 hours.

[0055] Specifically, the annealing furnace can be heated to a second temperature at a second heating rate. For example, the range of the second heating rate can be from 1°C / min to 3°C / min.

[0056] According to some embodiments of this disclosure, in order to prevent aluminum powder oxidation at high temperatures from affecting the sintering results, the second gas can be argon or a reducing protective atmosphere. Simultaneously, negative pressure sintering is continued to further eliminate organic matter in the high-temperature section, further reducing organic matter residue and improving sintering quality. The reducing protective gas can be, for example, hydrogen or alkanes. In addition to argon, it can also be an inert gas such as nitrogen or a mixture of argon and nitrogen.

[0057] S18. Cool the annealing furnace to the third temperature to complete the annealing.

[0058] After heating the annealing furnace to the second temperature and holding it at that temperature for the second time in step S16 above, a cooling process can be performed. The third temperature can be in the range of 60°C to 150°C, and the cooling rate of the annealing furnace to the third temperature can be in the range of 3°C / min to 8°C / min.

[0059] Preferably, the annealing furnace can be cooled to 60°C to 90°C, and the cooling method is furnace-side cooling.

[0060] Understandably, the sintered aluminum coil is then subjected to formation and anodizing treatments to obtain the final porous anode foil.

[0061] The above-described annealing method for anode foil in aluminum electrolytic capacitors offers several advantages. Firstly, the segmented negative pressure annealing process allows air at low temperatures to react with residual solvents, binders, and other organic matter within the coating. Under negative pressure, volatile organic compounds and their reaction products are promptly discharged, and continuous airflow carries away these waste gases, effectively preventing the formation of brown or black patches on the outer edge of the aluminum coil end face, which would negatively impact the sintering bond between aluminum powder and foil. Secondly, to prevent severe oxidation of aluminum powder at high temperatures from affecting the sintering effect, argon or an inert protective atmosphere is used, while continued negative pressure sintering further eliminates organic matter from the high-temperature stage, further reducing organic residue and aluminum foil quality defects. This also significantly improves the efficiency of organic matter removal; otherwise, brown and black patches from residual organic matter would be detrimental to user applications. Furthermore, the segmented negative pressure annealing method of this embodiment significantly shortens the annealing time, effectively reducing energy consumption by approximately 30%, decreasing energy consumption per unit output and the use of inert gas, thus lowering costs while simultaneously improving annealing efficiency and product quality. On the other hand, the porous anode foil prepared based on the scheme disclosed herein has strong bonding force between the sintered layer and the aluminum foil substrate, and consistent surface quality. The specific capacitance of the inner and outer ring products is small. When applied in the medium and high voltage formation field, it can not only obtain high capacitance, but also reduce the power consumption during formation, thus having higher product added value.

[0062] To better illustrate the details and effects of the annealing method for the anode foil of the aluminum electrolytic capacitor disclosed herein, the following descriptions of various embodiments and comparative examples of this disclosure are provided.

[0063] Example 1 The first step involves mixing and dispersing 60 wt% aluminum powder (particle size range, for example, 1 μm to 6 μm), 7.25 wt% binder and 32.5 wt% solvent evenly, and coating it uniformly onto an aluminum foil substrate with a thickness of 25 μm to 30 μm. After drying, the coating film has a thickness of 48 μm to 52 μm, resulting in a sintered anode foil with a thickness of 130 μm ± 2 μm. The foil is then wound into an aluminum coil using a winding machine.

[0064] The second step involves placing the aluminum coil in the annealing area for 24 to 48 hours, then covering the outer ring and end face of the aluminum coil with 3 to 5 layers of aluminum foil with a thickness of 0.06 mm to 0.2 mm, and sealing the gaps with aluminum tape.

[0065] The third step is to turn on the circulating fan and negative pressure vacuum pump of the heat treatment furnace, and control the pressure inside the furnace at 30,000 Pa to 40,000 Pa.

[0066] The fourth step involves raising the furnace gas temperature to 350°C at a rate of 1°C / min to 6°C / min under the aforementioned furnace pressure, and holding the temperature for 10 hours, during which air is continuously introduced.

[0067] Fifth step: stop the air introduction, evacuate the furnace until the pressure inside the furnace is ≤200Pa, continue to introduce argon or a reducing protective atmosphere, and at the same time maintain the pressure inside the furnace at 65000Pa to 70000Pa. Increase the furnace gas temperature of the annealing furnace to 655℃ at a heating rate of 1℃ / min to 3℃ / min, and hold at this temperature for 7 hours.

[0068] The sixth step is to reduce the furnace gas temperature of the annealing furnace to 90°C at a heating rate of 3°C / min to 8°C / min before unloading the furnace.

[0069] Figure 3 A photograph of the annealed surface of Embodiment 1 of this disclosure is shown.

[0070] Example 2 The first step involves mixing and dispersing 60 wt% aluminum powder (particle size range, for example, 1 μm to 6 μm), 7.25 wt% binder and 32.5 wt% solvent evenly, and coating it uniformly onto an aluminum foil substrate with a thickness of 25 μm to 30 μm. After drying, the coating film has a thickness of 48 μm to 52 μm, resulting in a sintered anode foil with a thickness of 130 μm ± 2 μm. The foil is then wound into an aluminum coil using a winding machine.

[0071] The second step involves placing the aluminum coil in the annealing area for 24 to 48 hours, then covering the outer ring and end face of the aluminum coil with 3 to 5 layers of aluminum foil with a thickness of 0.06 mm to 0.2 mm, and sealing the gaps with aluminum tape.

[0072] The third step is to turn on the circulating fan and negative pressure vacuum pump of the heat treatment furnace, and control the pressure inside the furnace between 40,000 Pa and 50,000 Pa.

[0073] The fourth step involves raising the furnace gas temperature to 515°C at a rate of 1°C / min to 6°C / min under the aforementioned furnace pressure, and holding it at that temperature for 8 hours, during which air is continuously introduced.

[0074] Fifth step: stop the air introduction, evacuate the furnace until the pressure inside the furnace is ≤200Pa, continue to introduce argon or a reducing protective atmosphere, and at the same time maintain the pressure inside the furnace at 70000Pa to 75000Pa. Increase the furnace gas temperature of the annealing furnace to 635℃ at a heating rate of 1℃ / min to 3℃ / min, and hold at that temperature for 9 hours.

[0075] The sixth step is to reduce the furnace gas temperature of the annealing furnace to 60°C at a heating rate of 3°C / min to 8°C / min before unloading the furnace.

[0076] Figure 4 A photograph of the annealed surface of Embodiment 2 of this disclosure is shown.

[0077] Comparative Example 1 This comparative example is basically the same as the above example one, except that: after coating and drying, it does not use vacuum sintering throughout the process, the pressure inside the furnace is ≤200Pa, and the holding time at 350℃ is extended to 24h, and the holding time at 655℃ is extended to 15h.

[0078] Figure 5 A photograph of the annealed surface of Comparative Example 1 of this disclosure is shown.

[0079] Comparative Example 2 This comparative example is basically the same as the above example 2, except that: after coating and drying, argon gas protection is used for sintering throughout the process.

[0080] Figure 6 A photograph of the annealed surface of Comparative Example 2 of this disclosure is shown.

[0081] In addition, the electrostatic capacity of the foils prepared in the above embodiments and comparative examples was tested, and the test results are shown in Table 1: Table 1

[0082] As can be clearly seen from Table 1, the anode foils prepared in Examples 1 and 2 have superior capacitance (Cap). In Example 2, the properties of the inner ring, outer ring, and middle section are more similar, and the properties of each part of the foil roll are more consistent. Therefore, the annealing process of Example 2 can be considered the preferred option.

[0083] In addition, through comparison Figures 3 to 6 It can be seen that the anode foil surfaces in Examples 1 and 2 have uniform color with almost no color difference. However, the anode foils in Comparative Examples 1 and 2 exhibit color difference bands formed by varying degrees of organic matter deposition at their two edges. This indicates that the surface quality after annealing using the scheme of this disclosure is superior.

[0084] In summary, the anode foil prepared by the annealing method for aluminum electrolytic capacitors according to the embodiments of this disclosure exhibits excellent specific capacitance and surface quality. The segmented negative voltage annealing method of this disclosure effectively reduces costs while improving annealing efficiency and product quality.

[0085] Furthermore, this disclosure also discloses an anode foil for an aluminum electrolytic capacitor, which is prepared based on the annealing method described above for the anode foil for an aluminum electrolytic capacitor.

[0086] It should be noted that although the steps of the method in this disclosure are described in a specific order in the accompanying drawings, this does not require or imply that the steps must be performed in that specific order, or that all the steps shown must be performed to achieve the desired result. Additional or alternative steps may be omitted, multiple steps may be combined into one step, and / or a step may be broken down into multiple steps.

[0087] Furthermore, the above figures are merely illustrative of the processes included in the method according to exemplary embodiments of this disclosure and are not intended to be limiting. It is readily understood that the processes shown in the above figures do not indicate or limit the temporal order of these processes. Additionally, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0088] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and examples are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the claims.

[0089] It should be understood that this disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is limited only by the appended claims.

Claims

1. An annealing method for anode foil used in aluminum electrolytic capacitors, characterized in that, include: The target aluminum coil is placed in an annealing furnace, and the target aluminum coil is obtained by at least winding the anode foil of an aluminum electrolytic capacitor; When the pressure inside the annealing furnace is controlled within a first pressure range and a first gas is continuously introduced into the annealing furnace, the annealing furnace is heated to a first temperature and held at that temperature for a first time. The first gas is introduced and then a vacuum is drawn to bring the pressure inside the annealing furnace below the pressure threshold. When the pressure inside the annealing furnace is controlled within the second pressure range and the second gas is continuously introduced, the annealing furnace is heated to the second temperature and held at that temperature for the second time. The annealing furnace is cooled to a third temperature to complete the annealing process; Wherein, both the first furnace pressure range and the second furnace pressure range are negative pressure ranges; the first gas is air, and the second gas is argon or a reducing protective gas.

2. The annealing method according to claim 1, characterized in that, The annealing method further includes: The aluminum electrolytic capacitor is wound up with anode foil to obtain a raw aluminum coil; The original aluminum coil is coated with a protective material to obtain the target aluminum coil.

3. The annealing method according to claim 1, characterized in that, The pressure range inside the first furnace is 3000 Pa to 80000 Pa, and the pressure range inside the second furnace is 35000 Pa to 90000 Pa.

4. The annealing method according to claim 1, characterized in that, The first temperature ranges from 300°C to 550°C, and the first time ranges from 6 h to 12 h.

5. The annealing method according to claim 4, characterized in that, Heating the annealing furnace to the first temperature includes: The annealing furnace is heated to a first temperature at a first heating rate; The first heating rate ranges from 1°C / min to 6°C / min.

6. The annealing method according to claim 1, characterized in that, The second temperature ranges from 600°C to 660°C, and the second time ranges from 3 h to 9 h.

7. The annealing method according to claim 6, characterized in that, Heating the annealing furnace to the second temperature includes: The annealing furnace is heated to the second temperature at a second heating rate; The second heating rate ranges from 1°C / min to 3°C / min.

8. The annealing method according to claim 1, characterized in that, The third temperature ranges from 60°C to 150°C, and the cooling rate of the annealing furnace to the third temperature ranges from 3°C / min to 8°C / min.

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