High resistivity stable silicon nitride composite ceramic and method of making same

Abstract

The application discloses a high-resistivity stability silicon nitride composite ceramic and a preparation method thereof, and belongs to the technical field of electric insulating ceramics, and overcomes the defect that the resistivity stability of an insulating support is poor in the prior art. The preparation method of the high-resistivity stability silicon nitride composite ceramic comprises the following steps: step 1, mixing silicon nitride powder, a sintering aid, a dispersing agent, a binder and water to prepare a slurry; the sintering aid comprises Y2O3, Al2O3 and MgO, and the mass ratio of Y2O3:Al2O3:MgO:silicon nitride is (0.01-0.15):(0.01-0.13):(0.01-0.1):1; step 2, preparing granulating powder; step 3, preparing a ceramic green body; step 4, removing glue; and step 5, performing first-step sintering under normal pressure, and then performing second-step sintering under nitrogen pressure. The high-resistivity stability silicon nitride composite ceramic prepared by the application has high resistivity stability.

Description

Technical Field

[0001] This invention belongs to the field of electrical insulating ceramics technology, specifically relating to a high resistivity stable silicon nitride composite ceramic and its preparation method. Background Technology

[0002] Silicon nitride ceramics possess high strength, high toughness, high thermal conductivity, and excellent resistance to oxidation, creep, and high resistivity, making them one of the most widely used structural ceramics. With the continuous development of powder metallurgy technology, silicon nitride, as a structural-functional integrated ceramic material with superior comprehensive performance, has received increasing attention. The combination of high strength, high thermal conductivity, and high resistivity in silicon nitride ceramics makes them potentially valuable for applications in ultra-high voltage power transmission.

[0003] DC through-wall bushings are crucial equipment in ultra-high voltage (UHVDC) transmission systems. Post insulators and basin insulators are the core components of the gas-insulated through-wall bushings, playing a key role in supporting the conductor rods. Post insulators are subjected to long-term high voltage, large temperature gradients, stress, and other multi-field coupling effects, severely testing their insulation performance. The high resistivity of silicon nitride post insulators and their resistivity stability under temperature gradients are essential guarantees for their safe operation. Currently used UHVDC gas-insulated posts commonly use composite materials of alumina and epoxy resin. When the temperature increases by 80°C from room temperature, their volume resistivity decreases by more than 90%. Under the same conditions, the volume resistivity of existing silicon nitride insulated posts also decreases by more than 60%. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defect of poor resistivity stability of the insulating pillar in the prior art, thereby providing a silicon nitride composite ceramic with high resistivity stability and its preparation method.

[0005] To this end, the present invention provides the following technical solution.

[0006] In a first aspect, the present invention provides a method for preparing high resistivity stable silicon nitride composite ceramics, comprising the following steps:

[0007] Step 1: Prepare a slurry by mixing silicon nitride powder, sintering aid, dispersant, binder and water;

[0008] The sintering aids include Y2O3, Al2O3, and MgO, with a mass ratio of Y2O3:Al2O3:MgO:silicon nitride of (0.01-0.15):(0.01-0.13):(0.01-0.1):1.

[0009] Step 2: Prepare granulated powder from the slurry by spray granulation;

[0010] Step 3: Press the granulated powder into shape to obtain a ceramic green body;

[0011] Step 4: Remove the adhesive from the ceramic green body;

[0012] Step 5: The ceramic green body after debinding is sintered under normal pressure for the first step, and then sintered under nitrogen pressure for the second step.

[0013] Furthermore, step 1 satisfies at least one of the following conditions:

[0014] (1) The silicon nitride powder is silicon nitride micron powder; preferably, the average particle size of the silicon nitride micron powder is 0.4-1.0 microns;

[0015] (2) The silicon nitride powder is α-Si3N4;

[0016] (3) The dispersant includes one or more of polyacrylamide, sodium hydroxymethyl cellulose, or Darvan CN;

[0017] (4) The adhesive includes one or more of polyvinyl alcohol, polyacrylic acid or polyethylene glycol;

[0018] (5) The mass ratio of the dispersant to silicon nitride powder is (0.01-0.25):1;

[0019] (6) The mass ratio of the binder to the silicon nitride powder is (0.03-0.26):1;

[0020] (7) The solid content of the slurry is 43-55 wt.%.

[0021] Furthermore, step 1 includes:

[0022] Silicon nitride powder, sintering aid, dispersant and water are premixed in a sand mill;

[0023] Then transfer the premixed material to a mixing tank, add the binder, and continue mixing;

[0024] Preferably, the mixture is premixed in a sand mill for 1 to 10 hours;

[0025] Preferably, the mixture is stirred in a mixing tank for 0.5 to 6 hours.

[0026] Furthermore, spray granulation also includes drying and sieving steps;

[0027] Furthermore, the drying conditions are drying at 110–150°C for 1–20 hours;

[0028] Furthermore, the sieving is performed through a 60-120 mesh sieve, and the material passing through the sieve is used as granulation powder.

[0029] Furthermore, in step 3, the pressing process includes at least one of dry pressing and cold isostatic pressing.

[0030] Furthermore, the pressure for dry pressing is 70–120 MPa, and the pressure is held for 5–10 minutes;

[0031] Furthermore, the pressure for cold isostatic pressing is 100–300 MPa, and the pressure is maintained for 5–10 minutes.

[0032] Furthermore, step 4 satisfies at least one of the following conditions:

[0033] (1) The ceramic green body is debonded in flowing air; preferably, the air flow rate is 0.5-4 L / min;

[0034] (2) The degreasing process includes: raising the temperature from room temperature to 300-450°C for 1-5 hours, holding the temperature for 1-6 hours, raising the temperature to 550-650°C for another 1-5 hours, holding the temperature for 0.5-5 hours, and then raising the temperature to 750-850°C for another 1-3 hours.

[0035] Furthermore, step 5 includes:

[0036] First, under normal pressure, heat to 1100-1500℃ at a rate of 5-10℃ / min and hold for 1-5 hours; then, purge with nitrogen, pressurize to 0.1-5MPa, heat to 1750-1900℃ at a rate of 1-3℃ / min, and hold for 1-5 hours.

[0037] Secondly, the present invention provides a high resistivity stable silicon nitride composite ceramic prepared according to the preparation method described above.

[0038] The powder on the sieve is mixed with solvent and then re-granulated.

[0039] The technical solution of this invention has the following advantages:

[0040] 1. A method for preparing high resistivity stable silicon nitride composite ceramics according to the present invention includes the following steps: Step 1, mixing silicon nitride powder, sintering aid, dispersant, binder and water to prepare a slurry; the sintering aid includes Y2O3, Al2O3 and MgO, and the mass ratio of Y2O3:Al2O3:MgO:silicon nitride is (0.01-0.15):(0.01-0.13):(0.01-0.1):1; Step 2, spray granulating the slurry to prepare granulated powder; Step 3, pressing the granulated powder into a ceramic green body; Step 4, removing the binder from the ceramic green body; Step 5, performing a first sintering under negative pressure on the debinded ceramic green body, followed by a second sintering under nitrogen pressure.

[0041] This invention employs a Y-Al-MgO ternary composite sintering aid. The Y2O3 sintering aid reduces the electrical conductivity barrier and improves temperature stability. Furthermore, Al2O3 and MgO are low-melting-point sintering aids, which can also lower the sintering temperature. Under the limited dosage of the ternary composite sintering aid of this invention, silicon nitride ceramics with uniform microstructure, high density, and stable resistivity can be prepared.

[0042] Spray granulation avoids sedimentation, separation, and re-agglomeration of components in the slurry, producing spherical powder with uniform particle size distribution while maintaining the original homogeneity of the slurry, thus ensuring the uniformity of the green body. Water-based spray granulation technology is environmentally friendly, with low production costs, simple operation, and high safety.

[0043] The granulated powder prepared by the spray granulation process of this invention is a silicon nitride spherical powder, which has the characteristics of high fluidity and good pressing performance. It can be used for green forming of large-size and complex-morphology silicon nitride ceramics, and the green body does not crack and has high density. As a result, the sintered silicon nitride composite ceramic has fewer micro-defects, which is beneficial to improving resistivity and resistivity stability.

[0044] This invention involves first sintering the debinded ceramic green body under normal pressure, followed by a second sintering under nitrogen pressure. The first stage of normal pressure sintering results in rapid shrinkage and increased density; the second stage of pressure sintering inhibits silicon nitride decomposition and further improves density.

[0045] The preparation method of this invention is simple, low-cost, more suitable for practical use, and has industrial application value.

[0046] (2) The high resistivity stable silicon nitride composite ceramic prepared by this invention has a density greater than 98% and a volume resistivity ≥1.0×10⁻⁶ at 80℃. 14 The resistivity of silicon nitride composite ceramic prepared by this invention is high, with a resistivity decrease rate of no more than 55% compared to room temperature and excellent resistivity stability at 80°C. Attached Figure Description

[0047] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0048] Figure 1 This is an SEM image of the fracture surface of the silicon nitride composite ceramic prepared in Example 1. Detailed Implementation

[0049] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0050] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0051] Example 1

[0052] This embodiment provides a method for preparing high resistivity stable silicon nitride composite ceramics, including the following steps:

[0053] (1) Take 10 kg of α-Si3N4 powder with an average particle size of 0.9 μm, 0.52 kg of yttrium oxide, 0.28 kg of alumina, 0.1 kg of magnesium oxide and 0.13 kg of polyacrylamide (average Mn 150000) as dispersant and put them into a sand mill. Use deionized water as the ball milling medium and premix for 2 hours. Then transfer the premixed product to a mixing tank, add 0.53 kg of polyethylene glycol (average Mn 8000) as binder, adjust the solid content to 55 wt.%, and continue stirring for 1 hour to obtain a slurry.

[0054] (2) The slurry was granulated by spray drying and then dried at 120°C for 10 hours. After drying, it was passed through a 60-mesh sieve and the sieve material was taken as granulation powder.

[0055] (3) The granulated powder is dry-pressed at 100MPa for 5 minutes, and then further cold isostatically pressed at 200MPa for 5 minutes to obtain a green body.

[0056] (4) The green body is debonded under flowing air with an air flow rate of 3L / min. The temperature is raised from room temperature to 400℃ in 3 hours, held for 5 hours, then raised to 600℃ in another 3 hours, held for 4 hours, and then raised to 800℃ in 1 hour without holding.

[0057] (5) The debinding green body is sintered in a gas pressure sintering furnace: First, under normal pressure, the temperature is raised to 1300℃ at a rate of 10℃ / min and held for 4 hours. Then, nitrogen is introduced, the pressure is increased to 1MPa, the temperature is raised to 1850℃, and held for 5 hours.

[0058] Figure 1 The image shown is a SEM image of the fracture surface of the silicon nitride composite ceramic prepared in Example 1. Figure 1It can be seen that the silicon nitride composite ceramics prepared by the present invention have uniform microstructure and high density.

[0059] Example 2

[0060] This embodiment provides a method for preparing high resistivity stable silicon nitride composite ceramics, including the following steps:

[0061] (1) Take 10 kg of α-Si3N4 powder with an average particle size of 0.4 μm, 0.5 kg of yttrium oxide, 0.25 kg of alumina, 0.15 kg of magnesium oxide, and 0.15 kg of polyacrylamide (average Mn 150000) as dispersant and put them into a sand mill. Use deionized water as the ball milling medium and premix for 2 hours. Then transfer the premixed product to a mixing tank, add 0.55 kg of polyethylene glycol (average Mn 8000) as binder, adjust the solid content to 52 wt.%, and continue stirring for 1 hour to obtain a slurry.

[0062] (2) The slurry was granulated by spray drying and then dried at 120°C for 10 hours. After drying, it was passed through a 100-mesh sieve and the sieve material was taken as granulation powder.

[0063] (3) The granulated powder is dry-pressed at 100MPa for 5 minutes and then further cold isostatically pressed at 200MPa for 5 minutes to obtain a green body.

[0064] (4) The green body is debonded under flowing air with an air flow rate of 2L / min. The temperature is raised from room temperature to 400℃ in 3 hours, held for 5 hours, then raised to 600℃ in another 3 hours, held for 4 hours, and then raised to 800℃ in 1 hour without holding.

[0065] (5) The debinding green body is sintered in a gas pressure sintering furnace: First, under normal pressure, the temperature is raised to 1400℃ at a rate of 10℃ / min and held for 4 hours. Then, nitrogen is introduced, the pressure is increased to 1MPa, the temperature is raised to 1850℃, and held for 5 hours.

[0066] Example 3

[0067] This embodiment provides a method for preparing high resistivity stable silicon nitride composite ceramics, including the following steps:

[0068] (1) Take 10 kg of α-Si3N4 powder with an average particle size of 0.7 μm, 0.5 kg wt.% of sintering aid yttrium oxide, 0.3 kg of alumina, 0.12 kg of magnesium oxide, and 0.15 kg of dispersant polyacrylamide (average Mn 150000) and put them into a sand mill. Use deionized water as the ball milling medium and premix for 2 hours. Then transfer the mixed slurry to a mixing tank, add 0.53 kg of binder polyethylene glycol (average Mn 8000), adjust the solid content to 50 wt.%, and continue stirring for 1 hour to obtain the slurry.

[0069] (2) The slurry was granulated by spray drying and then dried at 120°C for 10 hours. After drying, it was passed through a 60-mesh sieve and the sieve material was taken as granulation powder.

[0070] (3) The granulated powder is dry-pressed at 100MPa for 5 minutes and then further cold isostatically pressed at 250MPa for 5 minutes to obtain a green body.

[0071] (4) The green body is debonded under flowing air with an air flow rate of 2.5L / min. The temperature is raised from room temperature to 400℃ in 3 hours, held for 5 hours, then raised to 600℃ in another 3 hours, held for 4 hours, and then raised to 830℃ in 1 hour without holding.

[0072] (5) The green body after debinding is sintered in a gas pressure sintering furnace: first, under normal pressure, the temperature is raised to 1400℃ at a rate of 10℃ / min and held for 4 hours. Then, nitrogen is introduced, the pressure is increased to 1MPa, the temperature is raised to 1850℃, and held for 5 hours.

[0073] Comparative Example 1

[0074] This comparative example is basically the same as Example 1, except that yttrium oxide sintering aid was not used. The selected sintering aid was 0.28 kg of magnesium oxide and 0.1 kg of magnesium oxide.

[0075] Comparative Example 2

[0076] This comparative example is basically the same as Example 1, except that the amount of yttrium oxide sintering aid used is 0.08 kg.

[0077] Comparative Example 3

[0078] This comparative example is basically the same as Example 1, except that the two-step sintering process combining atmospheric pressure and gas pressure was not used. Only gas pressure sintering was used: nitrogen was introduced, the pressure was increased to 1 MPa, the temperature was raised to 1300°C at a rate of 10°C / min, held for 4 hours, and then the temperature was raised to 1850°C and held for 5 hours.

[0079] Test case

[0080] The silicon nitride composite ceramics prepared in Examples 1-3 and Comparative Examples 1-3 were thinned to 5 mm on both sides, polished on both sides, and then subjected to volume resistivity tests according to GB / T 1410-2006. The test results are shown in Table 1.

[0081] Table 1 Properties of Silicon Nitride Composite Ceramics

[0082]

[0083] As shown in Table 1, the silicon nitride ceramics prepared in the embodiments of the present invention have improved density and volume resistivity compared with those prepared in the comparative example, and the high-temperature stability of volume resistivity is also significantly improved.

[0084] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a high resistivity-stability silicon nitride composite ceramic for insulating supports, characterized in that, Includes the following steps: Step 1: Prepare a slurry by mixing silicon nitride powder, sintering aid, dispersant, binder and water; The sintering aids include Y2O3, Al2O3, and MgO, with a mass ratio of Y2O3:Al2O3:MgO:silicon nitride of (0.05-0.052):(0.028-0.03):(0.01-0.012):

1. Step 2: Prepare granulated powder from the slurry by spray granulation; Step 3: Press the granulated powder into shape to obtain a ceramic green body; Step 4: Remove the adhesive from the ceramic green body; Step 5: The ceramic green body after debinding is sintered under normal pressure for the first step, and then sintered under nitrogen pressure for the second step. Step 5 includes: first, heating to 1100-1500℃ at 5-10℃ / min under normal pressure, and holding at that temperature for 1-5 hours; Then nitrogen gas is introduced, the pressure is increased to 0.1~5MPa, and the temperature is increased to 1750~1900℃ at 1~3℃ / min, and held for 1~5 hours; The silicon nitride powder is silicon nitride micron powder; the average particle size of the silicon nitride micron powder is 0.4~1.0 micrometers; The mass ratio of the dispersant to silicon nitride powder is (0.01-0.25):1; The mass ratio of the binder to the silicon nitride powder is (0.03-0.26):1; The obtained high resistivity stable silicon nitride composite ceramic has a volume resistivity ≥1.0×10⁻⁶ at 80℃. 14 Ω·m.

2. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 1, characterized in that, Step 1 satisfies at least one of the following conditions: (1) The silicon nitride powder is α-Si3N4; (2) The dispersant includes one or more of polyacrylamide, sodium hydroxymethyl cellulose, or Darvan CN; (3) The adhesive includes one or more of polyvinyl alcohol, polyacrylic acid, or polyethylene glycol; (4) The solid content of the slurry is 43~55 wt.%.

3. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 1, characterized in that, Step 1 includes: Silicon nitride powder, sintering aid, dispersant and water are premixed in a sand mill; The premixed material is then transferred to a mixing tank, the binder is added, and mixing continues.

4. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 1, characterized in that, Spray granulation also includes drying and sieving steps.

5. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 4, characterized in that, The drying conditions are as follows: drying at 110~150℃ for 1~20 hours; And / or the sieving is performed through a 60-120 mesh sieve, and the material passing through the sieve is taken as granulation powder.

6. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 1, characterized in that, In step 3, the pressing process includes at least one of dry pressing and cold isostatic pressing.

7. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to claim 6, characterized in that, The pressure for dry pressing is 70~120MPa, and the pressure is held for 5~10 minutes; and / or The pressure for cold isostatic pressing is 100~300MPa, and the pressure is maintained for 5~10 minutes.

8. The method for preparing high resistivity stable silicon nitride composite ceramic for insulating supports according to any one of claims 1-7, characterized in that, Step 4 must satisfy at least one of the following conditions: (1) The ceramic green body is debonded in flowing air; (2) The degreasing process includes: raising the temperature from room temperature to 300-450°C for 1-5 hours, holding the temperature for 1-6 hours, raising the temperature to 550-650°C for another 1-5 hours, holding the temperature for 0.5-5 hours, and then raising the temperature to 750-850°C for another 1-3 hours.

9. A high resistivity stable silicon nitride composite ceramic for insulating pillars prepared by the preparation method according to any one of claims 1-8.

Images