CN111018508A

CN111018508A - Li-series microwave dielectric ceramic material and preparation method thereof

Info

Publication number: CN111018508A
Application number: CN201911407984.4A
Authority: CN
Inventors: 唐伟桓
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2020-04-17

Abstract

The invention relates to the technical field of microwave dielectric ceramics, in particular to a Li-series microwave dielectric ceramic material and a preparation method thereof; the invention comprises 32-34 mol% of Li₂CO₃、32%~34%SiO₂28% -34% of MgO and 0.01% -2.8% of CoO; the invention improves the dielectric constant of the Li-series microwave dielectric ceramic material, improves the temperature characteristic of the Li-series microwave dielectric ceramic material, and ensures that the Li-series microwave dielectric ceramic material has the excellent characteristics of ultra-low dielectric constant, high quality factor, low resonant frequency temperature coefficient, high relative density and low densification temperature; also provides a preparation method of the Li-series microwave dielectric ceramic material.

Description

Li-series microwave dielectric ceramic material and preparation method thereof

Technical Field

The invention relates to the technical field of microwave dielectric ceramics, in particular to a Li-series microwave dielectric ceramic material and a preparation method thereof.

Background

With the advent of the 5G communication era, the development of microwave materials has faced an unprecedented challenge, and electronic ceramic materials used for manufacturing microwave devices need to have a low dielectric constant, a high quality factor and a near-zero temperature coefficient of resonance frequency in consideration of signal transmission delay and loss during transmission.

The low temperature co-fired ceramic (LTCC) group is a packaging technology with a wide application prospect, and can realize large-scale three-dimensional integration of passive devices, thus being an important integration mode for reducing the size of a microwave assembly; considering that LTCC technology generally uses silver as an embedded electrode, where the melting point of silver is 961 ℃, the densification temperature of the main ceramic material should be lower than 961 ℃ in order to ensure the quality of the embedded circuit pattern.

In recent years, microwave dielectric ceramics have received much attention from the industry, such as Zn₂SiO₄、LiZnPO₄、Mg₂SiO₄And the like, the materials have great application value in the field of LTCC.

As is well known, silicates have a low dielectric constant, a high quality factor, a low temperature coefficient of resonance frequency, and good casting properties, and thus have been extensively studied; for example, Li₂MgSiO₄The ceramic material has an ultra-low dielectric constant (epsilon) when sintered at 1250 DEG C_r= 5.1), higher quality factor (Q × f =15,400 GHz), lower temperature coefficient of resonance frequency (τ f = -15.3 ppm/° c) and lower relative density (92%).

The dielectric property of the material is related to the intrinsic structure of the material, and the air holes generated in the manufacturing process of the material have close relation, the lower compactness can reduce the quality factor and the dielectric constant of the material and correspondingly deteriorate the electrical property of a device manufactured by the material, and in addition, the densification temperature of the dielectric ceramic material is higher than the melting point of silver, so the Li is higher than the melting point of the silver₂MgSiO₄The modification of the ceramic material should be started from reducing the sintering temperature and improving the relative density, and at present, Li₂MgSiO₄The dielectric constant of the base microwave dielectric ceramic material is high.

Disclosure of Invention

In order to overcome the above-mentioned disadvantages, the present invention aims to provide a Li-based microwave dielectric ceramic material, which improves the dielectric constant of the Li-based microwave dielectric ceramic material and the temperature characteristic of the Li-based microwave dielectric ceramic material, so that the Li-based microwave dielectric ceramic material has excellent characteristics of ultra-low dielectric constant, high quality factor, low temperature coefficient of resonance frequency, high relative density and low densification temperature; also provides a preparation method of the Li-series microwave dielectric ceramic material.

The technical scheme for solving the technical problem is as follows:

a Li-based microwave dielectric ceramic material comprises 32-34 mol% Li₂CO₃、32%~34%SiO₂28% -34% of MgO and 0.01% -2.8% of CoO.

As an improvement of the invention, 33.33% Li by mole percent is included₂CO₃、33.33%SiO₂30.68% MgO and 2.66% CoO.

As a further improvement of the invention, 33.33% Li in mole percent is included₂CO₃、33.33%SiO₂33.33% MgO and 0.01% CoO.

As a further improvement of the invention, 33.33% Li in mole percent is included₂CO₃、33.33%SiO₂31.84% MgO and 1.5% CoO.

A preparation method of a Li-based microwave dielectric ceramic material comprises the following steps:

step S1, weighing 32-34% Li by mol percentage₂CO₃、32%~34%SiO₂Mixing 28% -34% of MgO and 0.01% -2.8% of CoO to obtain powder;

step S2, mixing the powder with deionized water, and then placing the mixture into a ball mill for ball milling to obtain a primary ball grinding material;

step S3, drying the primary ball grinding material, sieving, and then pre-burning to obtain a pre-burning material;

step S4, mixing the pre-sintered material with deionized water, and putting the mixture into a ball mill for secondary ball milling to obtain a secondary ball grinding material;

step S5, adding an organic adhesive into the secondary ball grinding material, mixing to obtain granules, and pressing the granules into a cylindrical blank;

and step S6, placing the blank into a sintering furnace for low-temperature sintering to obtain the Li-based microwave dielectric ceramic material.

In step S2, the mixed mixture is ball-milled for 20 to 24 hours by using a planetary ball mill.

As a further improvement of the invention, in step S3, the pre-burning is carried out under the environment with the temperature of 800-850 ℃.

In a further improvement of the present invention, in step S4, the mixed mixture is ball milled for 20 to 24 hours by using a planetary ball mill.

As a further improvement of the present invention, in step S5, the organic binder accounts for 5% to 10% of the mass of the secondary ball grinding material.

As a further improvement of the invention, in step S6, sintering is carried out at a low temperature of 1000-1100 ℃ for 10-12 h.

The invention improves the dielectric constant of the Li-series microwave dielectric ceramic material, improves the temperature characteristic of the Li-series microwave dielectric ceramic material, and ensures that the Li-series microwave dielectric ceramic material has the excellent characteristics of ultra-low dielectric constant, high quality factor, low resonant frequency temperature coefficient, high relative density and low densification temperature; also provides a preparation method of the Li-series microwave dielectric ceramic material.

Drawings

For ease of illustration, the invention is described in detail in the following preferred embodiments and with reference to the accompanying drawings:

FIG. 1 is a block diagram of the steps of a process for preparing a Li-based microwave dielectric ceramic material according to the present invention;

FIG. 2 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of comparative example 1 after sintering;

FIG. 3 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of example 1 after sintering;

FIG. 4 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of example 2 after sintering;

FIG. 5 is a scanning electron micrograph of a Li-based microwave dielectric ceramic material of example 3 after sintering;

FIG. 6 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of example 4 after sintering.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As shown in FIGS. 1 to 6, the Li-based microwave dielectric ceramic material of the present invention comprises 32 to 34 mol% Li₂CO₃、32%~34%SiO₂28% -34% of MgO and 0.01% -2.8% of CoO.

In the present invention, Li is used₂CO₃、SiO₂MgO as main material, CoO as substitute material, and Co²⁺Ion-substituted part of Mg² ⁺The Li is contained in the main material and the substitute material by adjusting the dosage and the proportion of the main material and the substitute material, so that the dielectric constant of the Li-based microwave dielectric ceramic material is improved, the temperature characteristic of the Li-based microwave dielectric ceramic material is improved, and the Li-based microwave dielectric ceramic material has the excellent characteristics of ultra-low dielectric constant, high quality factor, low resonant frequency temperature coefficient, high relative density and low densification temperature.

Wherein, Co²⁺Has an ionic radius of 0.58 Å, Mg²⁺Has an ionic radius of 0.57 Å, is prepared from Co²⁺Ion-substituted Li₂MgSiO₄Mg in (1)²⁺The ions will change microscopically as follows: 1. mg (magnesium)²⁺The charge distribution symmetry around the ion is broken; 2. mg (magnesium)² ⁺The bonding number of the ions and the surrounding oxygen ions will be increased; 3. mg (magnesium)²⁺The bond length of the ions and the surrounding oxygen ions will decrease. Thus, with Co²⁺Ion-substituted Li₂MgSiO₄Mg in (1)²⁺The ions can distort the unit cell and thereby lower the densification temperature, allowing the material to achieve a higher degree of densification at lower sintering temperatures.

In the present invention, there is provided an embodiment of a Li-based microwave dielectric ceramic material comprising 33.33 mol% Li₂CO₃、33.33%SiO₂30.68 percent of MgO and 2.66 percent of CoO, wherein 2.66 percent of CoO is adopted to improve the microwave performance of the Li2MgSiO4 ceramic material, and the dielectric constant epsilon is tested_r5.02, a quality factor Qxf of 25,500GHz, and a relative density of 92.2%.

The present invention provides another embodiment of a Li-based microwave dielectric ceramic material comprising 33.33 mol% Li₂CO₃、33.33%SiO₂33.33% MgO and 0.01% CoO; measured as dielectric constant ε_r5.75, a quality factor Qxf of 26,500GHz, and a relative density of 95.2%.

The present invention provides yet another embodiment of a Li-based microwave dielectric ceramic material comprising 33.33 mol% Li₂CO₃、33.33%SiO₂31.84% MgO and 1.5% CoO. Measured as dielectric constant ε_r5.35, a quality factor Qxf of 25,800GHz, and a relative density of 94.2%.

More specifically, the Li-based microwave dielectric ceramic material comprises 33.33 percent of Li by mole percentage₂CO₃、33.33%SiO₂30.66% -33.32% of MgO and 0.01% -2.66% of CoO; the molar ratio of MgO to CoO is (1:0.01) - (0.092:0.08), wherein too high or too low the CoO dosage can not improve Li better₂MgSiO₄The microwave properties of the ceramic material; too low an amount of Li₂MgSiO₄The microwave performance of the ceramic is not greatly improved, the use amount is too high, and the lattice distortion caused by the substitution of a large amount of ions can deteriorate the microwave performance of the Li-based microwave dielectric ceramic material.

The Li-based microwave dielectric ceramic material has the excellent characteristics of low dielectric constant, high quality factor, high relative density and low densification temperature, and can improve sintering characteristics, microscopic morphology and electrical properties at a low sintering temperature of 1000-1100 ℃.

As shown in FIG. 1, the present invention provides a method for preparing a Li-based microwave dielectric ceramic material, comprising the following steps:

In the preparation method of the Li-based microwave dielectric ceramic material, the sintering characteristic, the microscopic morphology and the electrical property of the Li2MgSiO4 microwave dielectric ceramic are improved at a low sintering temperature.

In the step S2, ball milling is carried out on the mixed mixture for 20-24 hours by using a planetary ball mill; specifically, the primary ball milling is carried out by mixing the mixed powder with deionized water and then putting the mixture into a ball mill, wherein the primary ball milling time is 20-24 hours, and preferably 24 hours.

In step S3, pre-burning is carried out in the environment with the temperature of 800-850 ℃; specifically, the pre-sintering is to dry and sieve the primary ball grinding material obtained after primary ball milling, and then to heat up and pre-sinter and preserve heat; the pre-sintering temperature is 800-850 ℃, for example, the pre-sintering temperature is 800 ℃, 810 ℃, 820 ℃, 830 ℃, 840 ℃ and 850 ℃; the heating rate is 2-5 ℃/min; the temperature for heat preservation is 810-840 ℃, for example, the temperature for heat preservation is 810 ℃, 815 ℃, 820 ℃, 825 ℃, 830 ℃, 835 ℃ and 840 ℃, most preferably 830 ℃, and the time for heat preservation is 10 h-12 h, for example, the time for heat preservation is 10h, 10.2h, 10.4h, 10.6h, 10.8h, 11.0h, 11.2h, 11.4h, 11.6h, 11.8h and 12.0 h.

In the step S4, performing ball milling on the mixed mixture for 20-24 hours by using a planetary ball mill; specifically, the secondary ball milling is carried out by mixing the pre-sintered material with deionized water and then putting the mixture into a ball mill; the time of the secondary ball milling is 20-24 h.

In the step S5, the organic adhesive accounts for 5% -10% of the mass of the secondary ball grinding material; specifically, the granulation molding is to add organic adhesive into the secondary ball-milled material obtained after secondary ball milling, granulate after mixing, and press the granular powder into blanks by a press and a mold; the amount of the organic binder accounts for 5-10% of the mass of the secondary ball grinding material, for example, the amount of the organic binder accounts for 5%, 6%, 7%, 8%, 9%, 10% of the mass of the secondary ball grinding material; the blank is a cylindrical blank, the radius of the cylindrical blank is 0.5-0.7 cm, and the height of the cylindrical blank is 0.5-0.7 cm

In the step S6, sintering at a low temperature of 1000-1100 ℃ for 10-12 h; specifically, the low-temperature sintering is to place the granulated and molded blank in a sintering furnace for low-temperature sintering; preferably, the low-temperature sintering temperature is 1000-1100 ℃, for example, the low-temperature sintering temperature is 1000 ℃, 1020 ℃, 1040 ℃, 1060 ℃, 1080 ℃ and 1100 ℃, and more preferably 1050 ℃; the low-temperature sintering time is 10 h-12 h, for example, the low-temperature sintering time is 10h, 10.2h, 10.4h, 10.6h, 10.8h, 11.0h, 11.2h, 11.4h, 11.6h, 11.8h and 12.0 h; the atmosphere of the low-temperature sintering is air, and the heating rate of the low-temperature sintering is 2-5 ℃/min.

The invention provides an implementation mode of a preparation method of a Li-series microwave dielectric ceramic material, which comprises the following steps:

1) weighing 33.33% of Li in terms of molar percentage₂CO₃33.33% SiO₂30.66% -33.32% of MgO and 0.01% -2.66% of CoO;

2) mixing the powder material weighed in the step 1) with deionized water, and carrying out primary ball milling in a planetary ball mill for 20-24 hours;

3) drying and screening the primary ball-milled material obtained in the step 2), then pre-burning at the temperature of 800-850 ℃, wherein the temperature rise rate is 2-5 ℃/min, and the temperature is maintained at the temperature of 810-840 ℃ for 10-12 hours;

4) mixing the pre-sintered material obtained after the pre-sintering in the step 3) with deionized water, and performing secondary ball milling in a planetary ball mill for 20-24 hours;

5) adding the secondary ball grinding material obtained in the step 4) into an organic adhesive accounting for 5-10% of the mass of the secondary ball grinding material, uniformly mixing, granulating, pressing granular powder into a cylindrical blank by using a press and a die, wherein the radius of the cylindrical blank is 0.5-0.7 cm, and the height of the cylindrical blank is 0.5-0.7 cm;

6) and (3) placing the blank obtained in the step 5) in a sintering furnace in air atmosphere for low-temperature sintering, wherein the low-temperature sintering temperature is 1000-1100 ℃, the low-temperature sintering time is 10-12 h, and the heating rate is 2-5 ℃/min, so that the Li-based microwave dielectric ceramic material with the ultralow dielectric constant is obtained.

In the invention, the lattice distortion generated by the difference of polarizability and radius of the substituted ions and the substituted ions can reduce the densification temperature; the invention is based on the preference for high-purity Li₂CO₃MgO, CoO and SiO₂The optimal scheme for preparing the Li-based microwave dielectric ceramic material with the ultralow dielectric constant is determined by adjusting the proportion of the raw materials; on the premise of optimizing the formula and the preparation process of the powder material, the low-temperature sintering of the Li-system microwave dielectric ceramic material with high quality factor, ultralow dielectric constant and high density and ultralow dielectric constant is realized by combining the sintering curve with high density and uniform particle size.

The Li-based microwave dielectric ceramic material is applied to the field of low-temperature co-fired ceramics, and the high quality factor of the Li-based microwave dielectric ceramic material can effectively reduce the loss of a microwave device to signals during working, so the Li-based microwave dielectric ceramic material has great significance for improving the performance of the microwave device; moreover, the ultralow dielectric constant of the Li-based microwave dielectric ceramic material can effectively reduce the delay time of signal transmission, thereby improving the real-time property of signals; in addition, the invention uses Co²⁺The replacement of ions effectively reduces the intrinsic densification temperature of the material, so that ultralow-temperature sintering can be realized by adding a smaller amount of sintering aid, and the deterioration of electrical properties caused by the use of excessive sintering aid is reduced.

The present invention provides example 1:

the composition of the Li-based microwave dielectric ceramic material of example 1 is shown in Table 1.

The method for preparing the Li-based microwave dielectric ceramic material of the embodiment 1 comprises the following steps:

1) preparing materials according to the material composition and the proportion in the table 1;

2) mixing the powder material weighed in the step 1) with deionized water, and carrying out primary ball milling in a planetary ball mill for 24 hours;

3) drying and screening the primary ball-milled material obtained in the step 2), then presintering at the temperature of 830 ℃, presintering at the heating rate of 5 ℃/min, and keeping the temperature for 11 hours;

4) mixing the pre-sintered material obtained after the pre-sintering in the step 3) with deionized water, and performing secondary ball milling in a planetary ball mill for 24 hours;

5) adding the secondary ball grinding material obtained in the step 4) into an organic adhesive accounting for 8% of the mass of the secondary ball grinding material, uniformly mixing, granulating, pressing granular powder into a cylindrical blank by using a press and a die, wherein the radius of the cylindrical blank is 0.6cm, and the height of the cylindrical blank is 0.6 cm;

6) and (3) placing the blank obtained in the step 5) in a sintering furnace in the air atmosphere for low-temperature sintering, wherein the low-temperature sintering temperature is 1050 ℃, the low-temperature sintering time is 11h, and the heating rate is 5 ℃/min, so that the Li-based microwave dielectric ceramic material with the ultralow dielectric constant is obtained.

The present invention provides example 2:

the composition of the Li-based microwave dielectric ceramic material of example 2 is shown in Table 1.

The preparation method of the Li-based microwave dielectric ceramic material of example 2 is the same as that of example 1.

The present invention provides example 3:

the composition of the Li-based microwave dielectric ceramic material of example 3 is shown in Table 1.

The preparation method of the Li-based microwave dielectric ceramic material of example 3 is the same as that of example 1.

The present invention provides example 4:

the composition of the Li-based microwave dielectric ceramic material of example 4 is shown in Table 1.

The preparation method of the Li-based microwave dielectric ceramic material of example 4 is the same as that of example 1.

The Li-based microwave dielectric ceramic materials of examples 1 to 4 and comparative examples 1 to 2 were prepared in the composition ratios shown in Table 1.

TABLE 1

The present invention provides example 5:

this example 5 is different from example 1 in that the sintering temperature in the low temperature sintering is 1020 ℃ in the preparation method, and the other steps are the same as example 1.

The present invention provides example 6:

the present example 6 is different from example 1 in that the sintering temperature in the low-temperature sintering is 1080 ℃ in the preparation method, and the rest is the same as example 1.

The present invention provides example 7:

this example 7 is different from example 1 in that the temperature of the pre-firing in the preparation method was 800 ℃, and the other steps were the same as those of example 1.

The present invention provides example 8:

this example 8 is different from example 1 in that the temperature of the pre-firing in the preparation method was 850 ℃, and the other steps were the same as those of example 1.

The present invention provides comparative example 1:

the compounding was carried out according to the composition of comparative example 1 in Table 1, which is different from example 1 in that CoO was not contained in the raw material and that the sintering temperature was 1050 ℃.

The present invention provides comparative example 2:

comparative example 2 is different from example 1 in that the molar percentage of CoO is 5%, the molar percentage of MgO is 28.33%, and other components and preparation methods are the same as those of example 1.

The present invention provides comparative example 3:

comparative example 3 is different from example 1 in that only one ball milling is used in the preparation method, and the rest is the same as example 1.

The present invention provides comparative example 4:

comparative example 4 is different from example 1 in that the preparation method is the same as example 1 except that the pre-firing is not performed.

The present invention provides comparative example 5:

comparative example 5 is different from example 1 in that the temperature of the pre-firing in the preparation process was 500 deg.c, and the rest was the same as example 1.

The present invention provides comparative example 6:

comparative example 6 is different from example 1 in that the temperature of the pre-firing in the preparation process is 1000 c, and the rest is the same as example 1.

The present invention provides comparative example 7:

comparative example 7 is different from example 1 in that the sintering temperature in the preparation process is 1000 deg.C, and the rest is the same as example 1.

The present invention provides comparative example 8:

comparative example 8 is different from example 1 in that the sintering temperature in the preparation process was 1300 c, and the rest was the same as example 1.

The Li-based microwave dielectric ceramic materials of examples 1 to 9 and comparative examples 1 to 8 were subjected to performance tests including electrical performance tests, microstructure tests, and the like, and the test results are shown in Table 2.

Wherein the dielectric constant ε_rThe quality factor Qxf was tested using an Agilent N5230A Network Analyzer (300 MHz-20 GHz); the relative density was measured by the archimedes drainage method; testing a scanning electron microscope image (SEM) by using JEOL JSM-6490 LV; x-ray diffraction (XRD) was tested using DX-2700 (Haoyuan Co.).

TABLE 2

As can be seen from examples 1 to 4, the dielectric constant, the relative density and the Qxf value of the Li-based microwave dielectric ceramic material show a tendency to increase and then decrease with the increase of the CoO amount at 1050 ℃, and the ceramic material shows the best properties in example 2; as can be seen from examples 6-9, when the pre-sintering temperature is 830 ℃ and the secondary sintering temperature is 1050 ℃, the material performance is optimal; as can be seen from comparative examples 1 to 8, the microwave dielectric ceramic material prepared by the process of example 1 has the best performance, and the microwave dielectric performance is improved.

FIG. 2 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of comparative example 1 after sintering; as can be seen from FIG. 2, after sintering at 1050 deg.C, the Li-based microwave dielectric ceramic material prepared without CoO has many pores and low densification degree.

FIG. 3 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of example 1 after sintering; FIG. 4 is a scanning electron micrograph of the Li-based microwave dielectric ceramic material of example 2 after sintering; FIG. 5 is a scanning electron micrograph of a Li-based microwave dielectric ceramic material of example 3 after sintering; FIG. 6 is a scanning electron micrograph of a Li-based microwave dielectric ceramic material of example 4 after sintering; as can be seen from fig. 3 to 6, as the CoO content increases, the densification degree of the material increases first and then decreases, and the corresponding pores in the material show a tendency to decrease first and then increase.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims

1. The Li-based microwave dielectric ceramic material is characterized by comprising the following components in percentage by mole

32%~34%Li₂CO₃、32%~34%SiO₂28% -34% of MgO and 0.01% -2.8% of CoO.

2. The Li-based microwave dielectric ceramic material as claimed in claim 1, comprising 33.33 mol% Li₂CO₃、33.33%SiO₂30.68% MgO and 2.66% CoO.

3. The Li-based microwave dielectric ceramic material as claimed in claim 1, comprising 33.33 mol% Li₂CO₃、33.33%SiO₂33.33% MgO and 0.01% CoO.

4. The Li-based microwave dielectric ceramic material as claimed in claim 1, comprising 33.33 mol% Li₂CO₃、33.33%SiO₂31.84% MgO and 1.5% CoO.

5. A preparation method of a Li-based microwave dielectric ceramic material is characterized by comprising the following steps:

6. The method for preparing a Li-based microwave dielectric ceramic material as claimed in claim 5, wherein in step S2, the mixed mixture is ball milled for 20-24 h by using a planetary ball mill.

7. The method for preparing a Li-based microwave dielectric ceramic material as claimed in claim 5, wherein in step S3, pre-firing is performed at a temperature of 800 ° -850 °.

8. The method for preparing a Li-based microwave dielectric ceramic material as claimed in claim 5, wherein in step S4, the mixed mixture is ball milled for 20-24 h by using a planetary ball mill.

9. The method for preparing a Li-based microwave dielectric ceramic material as claimed in claim 5, wherein in step S5, the organic binder accounts for 5-10% by mass of the secondary ball grinding material.

10. The method for preparing a Li-based microwave dielectric ceramic material as claimed in claim 5, wherein in step S6, the sintering is performed at a low temperature of 1000 ° -1100 ° for 10-12 h.