CN111410540A - Preparation method of porous silicon nitride ceramic with directional pore structure - Google Patents
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Abstract
The invention relates to a preparation method of porous silicon nitride ceramics with a directional pore structure, which comprises the following steps: (1) dispersing silicon powder, silicon nitride powder, a sintering aid and a binder in tert-butyl alcohol to obtain ceramic slurry; (2) pouring the obtained ceramic slurry into a mold, freezing and solidifying under a one-way cold source, and drying and removing gel to obtain a ceramic precursor; (3) and igniting the ceramic precursor embedded by adopting embedded powder consisting of silicon powder and silicon nitride powder in a nitrogen atmosphere to initiate self-propagating synthesis to obtain the porous silicon nitride ceramic with the directional pore structure.
Description
Technical Field
The invention relates to a method for preparing porous silicon nitride ceramics with a directional pore structure quickly and at low cost, belonging to the field of preparation of porous silicon nitride ceramics.
Background
The directional pore structure porous silicon nitride ceramic is a structure-function integrated ceramic with excellent performance. On one hand, the ceramic material has excellent properties of high strength, high modulus, corrosion resistance, high temperature resistance and the like; on the other hand, the method can be used for designing pore parameters such as porosity, pore size distribution and the like, so that the method has wide application prospects in the fields of high-temperature filtration, purification and separation, catalytic carriers, sound insulation and decoupling and the like. The preparation of the ceramic precursor with the oriented pore structure and the subsequent sintering thereof are basic steps for preparing the porous silicon nitride ceramic with the oriented pore structure. For the preparation of the green body with the directional pore structure, a sacrificial template method, a sodium alginate gel method, a freeze drying method and the like are mainly adopted; for the sintering of silicon nitride ceramics, there are mainly gas pressure sintering, pressureless sintering and reaction sintering.
Chinese patent document 1 (publication No. CN104529523A) discloses a method for preparing a directional-pore porous silicon nitride ceramic by normal pressure sintering using carbon fibers as a pore-forming agent; chinese patent document 2 (publication No. CN104926355A) discloses a method for preparing oriented porous silicon nitride ceramics based on gelatin solution freeze-drying technology and normal pressure sintering; chinese patent document 3 (publication No. CN105645967A) discloses a method for preparing highly oriented through-hole porous silicon nitride ceramics by using a reaction in which silicon powder reacts with water to generate hydrogen gas to form oriented through-holes. With these methods disclosed so far, a green compact of an oriented pore structure can be efficiently prepared, but the sintering time is long, the energy consumption is large, and large-size sample preparation is difficult due to the complicated equipment.
Disclosure of Invention
Therefore, the invention provides a rapid and low-cost preparation method of porous silicon nitride ceramics with a directional pore structure, which comprises the following steps:
(1) dispersing silicon powder, silicon nitride powder, a sintering aid and a binder in tert-butyl alcohol to obtain ceramic slurry;
(2) pouring the obtained ceramic slurry into a mold, freezing and solidifying under a one-way cold source, and drying and removing gel to obtain a ceramic precursor;
(3) and igniting the ceramic precursor embedded by adopting embedded powder consisting of silicon powder and silicon nitride powder in a nitrogen atmosphere to initiate self-propagating synthesis to obtain the porous silicon nitride ceramic with the directional pore structure.
In the invention, the directional porous silicon nitride ceramic is prepared by combining directional freeze-drying with self-propagating reaction for the first time. Specifically, silicon powder, silicon nitride powder, a sintering aid and a binder are dissolved in tert-butyl alcohol to prepare ceramic slurry. Due to the characteristic of needle-shaped crystals of the selected solvent tert-butyl alcohol, in the process of freezing and solidifying under a one-way cold source, the tert-butyl alcohol is gradually crystallized and can well directionally grow along the temperature gradient, and finally a blank with a directional pore structure is formed. And (4) carrying out glue discharging on the obtained blank to remove the binder. Then igniting in nitrogen atmosphere to initiate self-propagating synthesis to obtain the directional porous silicon nitride ceramic. The method provided by the invention is simple to operate, is rapid in reaction, can greatly reduce energy consumption and preparation cost, and is convenient for large-scale popularization and application.
Preferably, in the step (1), the particle size of the silicon powder is 0.5-5 μm, the particle size of the silicon nitride powder is 0.5-5 μm, and the α phase content is 90-99 wt%.
Preferably, in the step (1), the mass ratio of the silicon powder to the silicon nitride powder is 6-3.5: 4 to 6.5.
Preferably, in the step (1), the sintering aid is at least one selected from yttrium oxide, ytterbium oxide, cerium oxide and lanthanum oxide.
Preferably, in the step (1), the addition amount of the sintering aid is 1 to 8wt%, preferably 2 to 5wt%, of the total mass of the silicon powder and the silicon nitride powder.
Preferably, in the step (1), the binder is polyvinyl butyral PVB, and the adding amount of the binder is 1-5 wt% of the total mass of the silicon powder and the silicon nitride powder.
Preferably, in the step (1), the solid content of the ceramic slurry is calculated according to the silicon nitride formed by complete nitridation of silicon powder, and the amount of the tert-butyl alcohol is such that the solid content of the ceramic slurry is 10-50 vol.%, preferably 20-40 vol.%.
Preferably, in the step (2), before the ceramic slurry is poured into the mold, a defoaming agent is added and vacuum defoaming is carried out, wherein the defoaming agent is 1-octanol. If the foam is not removed, the foam of the solution can form round holes during freezing and solidification, so that more defects are formed, the orientation effect is poor, and the compression strength is further reduced.
Preferably, in the step (2), the freezing temperature of the one-way cold source is-18 to-80 ℃.
Preferably, in the step (2), the temperature of the rubber discharge is 500-600 ℃, and the time is 1-3 hours.
Preferably, in the step (3), the pressure of the nitrogen atmosphere is 2 to 20MPa, and preferably 3 to 15 MPa.
On the other hand, the invention also provides the directional pore structure porous silicon nitride ceramic prepared by the preparation method, the directional pore structure porous silicon nitride ceramic has the microstructure of long rod-shaped mutually-overlapped crystal grains, the pore structure is a directional pore, the pore diameter is 10-200 mu m, and the phase composition is pure β -Si3N4。
Preferably, the total porosity of the porous silicon nitride ceramic with the oriented pore structure is 50-90%, the axial compressive strength is 3-120 MPa, and the transverse compressive strength is 1-60 MPa.
Has the advantages that:
compared with the disclosed preparation method of the oriented porous silicon nitride ceramic, the method selects the silicon powder as one of the raw material powders, but avoids the time-consuming and energy-consuming sintering method of reaction sintering; self-propagating reaction sintering is adopted instead, and the heat released by silicon powder nitridation is used as a heat source for silicon nitride sintering. The preparation process is simple, and the sintering time and the equipment energy consumption are obviously reduced, so that the preparation cost is greatly reduced. The sintered sample has small shrinkage, obvious directional pore structure and excellent mechanical property.
Drawings
FIG. 1 is a comparison of the samples of example 1 before and after sintering, from which it can be seen that the sample sizes before and after sintering match well and the sample shrinkage is small;
FIG. 2 is the XRD contrast before and after sintering of the sample of example 1, from which it can be seen that the sample is completely nitrided and the phase composition of the sample after sintering is β -Si3N4；
FIG. 3 is a scanning electron microscope image of the axial and transverse micro-topography of the porous silicon nitride ceramic material prepared in example 1, wherein a, c and d in FIG. 3 are micro-topography images of a sample at different magnifications in the direction perpendicular to the pores, and b in FIG. 3 is a micro-topography image in the direction parallel to the hollow; as can be seen, the holes are clearly and uniformly aligned in the vertical direction, and the alignment grooves are clearly aligned in the parallel direction, indicating that a uniform alignment hole structure is formed in the sample.
Detailed Description
The present invention is further illustrated by the following examples, which are to be understood as merely illustrative and not restrictive.
In the disclosure, the directional pore structure porous silicon nitride ceramic is prepared by mixing silicon powder, silicon nitride diluent (silicon nitride powder), sintering aid and binder as raw materials by using tert-butyl alcohol as a solvent to prepare ceramic slurry, then performing freeze drying molding, and performing subsequent self-propagating reaction.
The following is an exemplary description of the preparation method of the porous silicon nitride ceramic with the oriented pore structure provided by the invention.
And carrying out ball milling and mixing on the silicon powder, the silicon nitride diluent, the sintering aid and the binder in a tert-butyl alcohol solvent to obtain uniform ceramic slurry. Wherein the rotation speed of ball milling mixing is 200-300 r/min, and the time is 1-3 hours, such as 2 hours.
As a preferable scheme, the dosage of the tert-butyl alcohol is calculated according to the theoretical solid content of the ceramic slurry after silicon powder is completely nitrided to form silicon nitride, and the solid content is designed to be 10-50 vol.%. Firstly, the difference between the theoretical porosity of the sample and the actual porosity of the sample can be clearly seen, and the analysis is convenient. Secondly, in the solid content range, the ceramic precursor has certain strength after rubber removal, and is convenient to process; and directional holes in the sample are uniformly distributed, and the defects are few. If the solid content is too high, the solvent is less, the uniform stirring is difficult, even fluid slurry cannot be formed, and a uniform directional pore structure cannot be obtained. If the solid content is too low, the defects of the frozen green body are large, and the green body after debonding has low strength and is easy to crack. The solid content of the ceramic slurry can be further preferably selected to be 20-40 vol.%, and in the preferred range, the sample has few defects and uniform pore diameter.
And pouring the ceramic slurry into a mold, and performing freeze drying molding and glue discharging under a one-way cold source (the freezing temperature is-18 to-80 ℃) to prepare the ceramic precursor with the directional pore structure. Wherein the formation mechanism of the directional hole is as follows: the solvent in the solution is gradually crystallized and grows along the temperature gradient under the action of a one-way cold source. The growth of the ice crystals extrudes the ceramic particles to form hole walls, and the directional ice crystals are sublimated and discharged after being dried to form directional holes. The tert-butyl alcohol has the characteristic of needle-shaped crystals, so that the tert-butyl alcohol has better advantages in the formation of directional pores. If the drying is carried out by using water or additionally adding other solvents such as water, the directional hole effect is not good. Wherein the temperature of the binder removal can be 500-600 ℃, and the time can be 1-3 hours. For example, the binder removal may be at 600 ℃ for 2 hours.
And sintering the ceramic precursor by self-propagating reaction to prepare the porous silicon nitride ceramic with the directional pore structure. The self-propagating reaction sintering is maintained by the intense reaction heat release of the silicon powder nitriding reaction, and is generally performed in a high-pressure nitrogen atmosphere. The pressure of the high-pressure nitrogen is 2MPa to 20MPa, preferably 3 MPa to 15 MPa. Preferably, the mass ratio of the silicon powder to the silicon nitride diluent in the ceramic precursor is 6-3.5: 4-6.5. If the silicon powder content in the ceramic precursor is too low, the reaction temperature is too low to complete the sintering process, and even the reaction may not be maintained. If the silicon powder content in the ceramic precursor is too high, the sample is easy to crack, shrink seriously, and over-sinter.
In an optional implementation mode, the ceramic precursor and the embedded powder after debonding (or referred to as binder removal) are placed in a graphite boat in a self-propagating reaction kettle, high-pressure nitrogen is filled in the graphite boat, the graphite boat is ignited, and the porous silicon nitride ceramic with the oriented pore structure is obtained after cooling. The used buried powder and ceramic slurry contain silicon powder and silicon nitride diluent in the same mass ratio, but do not contain sintering aid. Namely, the mass ratio of the silicon powder to the silicon nitride diluent in the embedded powder is also 6-3.5: 4-6.5.
In an alternative embodiment, since the ceramic slurry is prepared by ball milling and mixing, it is easy to generate a large amount of bubbles, and further defoaming treatment is required for subsequent directional freezing. Specifically, 1-2 drops of a defoaming agent is added into the obtained ceramic slurry, and vacuum defoaming is performed.
As a detailed preparation example of the directional pore structure porous silicon nitride ceramic, the method comprises the following steps: (1) weighing sintering aids according to 1-8% of the mass fraction of the ceramic powder, weighing PVB (polyvinyl butyral) binders according to 1-5% of the mass fraction of the ceramic powder, adding corresponding mass of tert-butyl alcohol as a solvent, weighing silicon nitride grinding balls according to a ball-to-material ratio of 2:1, and placing the materials in a polytetrafluoroethylene tank for ball milling and uniformly mixing for 2 hours; preferably, the content of the sintering aid is 2-5%. (2) Adding 1-2 drops of 1-octanol into the ball-milled slurry, carrying out vacuum defoaming, pouring the defoamed slurry into a polytetrafluoroethylene mold with an aluminum plate as the bottom surface, surrounding the periphery and the top of the mold by asbestos thermal insulation boards, and then placing the mold in a freezer at-18 to-80 ℃ for freezing and curing. (3) And drying the completely frozen and solidified sample for 24h under low pressure to obtain a green body, placing the green body in a muffle furnace, slowly heating to 600 ℃ for glue removal, wherein the heating rate is 2 ℃/min, and keeping the temperature for 2h to obtain the ceramic precursor. (4) And putting the ceramic precursor sample after glue removal and the embedded powder into a graphite boat in a self-propagating reaction kettle, filling 2-20 MPa high-pressure nitrogen, igniting, and cooling to obtain the porous silicon nitride ceramic with the directional pore structure. Preferably, the nitrogen pressure is 3-15 MPa.
In conclusion, the invention combines the advantages of directional freeze drying and self-propagating reaction, can obviously reduce the preparation cost of the porous silicon nitride ceramic with the directional pore structure, and promotes the large-scale production and application of the porous silicon nitride ceramic.
And (3) performance testing:
in the invention, the theoretical porosity of the porous silicon nitride ceramic with the oriented pore structure is calculated to be 50-90% through a formula (the theoretical porosity is 1-designed solid content). The measured porosity of the material is 50-90% by adopting an Archimedes drainage method. The shrinkage rate of the sample is 0.3-2% by measuring the size of the sample before and after sintering. The axial compression strength of the material is 3-120 MPa and the transverse compression strength of the material is 1-60 MPa by adopting a universal testing machine.
The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.
Example 1
Respectively weighing 35g of silicon powder, 65g of silicon nitride diluent, 2g of yttrium oxide powder, 92.6g of tertiary butanol (calculated according to 35 vol.% of theoretical solid content), 2g of PVB binder and 200g of silicon nitride grinding balls, adding the above six materials into a ball-milling tank made of polytetrafluoroethylene, carrying out ball milling and uniformly mixing for 2 hours on a planetary ball mill at the speed of 300r/min to prepare uniform and stable mixture slurry, adding 1-2 drops of 1-octanol into the prepared slurry, placing the slurry into a vacuum bubble removal machine for bubble removal for 20 minutes, pouring the slurry after bubble removal into a polytetrafluoroethylene mold with the bottom surface being 50cm and × 50cm of an aluminum plate, surrounding the periphery and the top of the mold with a cotton heat insulation plate, placing the mold into a freezer at-18 ℃ for freezing and curing for 12 hours, taking out the completely cured sample from the mold, placing the sample into a vacuum drying machine for drying for 24 hours, controlling the air pressure of the drying box within 1000pa, placing the dried blank into a muffle furnace for binder removal, raising the temperature to 600 hours at the speed of 2 minutes, cooling the sample, placing the sample into a ceramic precursor, placing the ceramic buried powder into a ceramic buried reactor, carrying out a ceramic diffusion reaction furnace, and introducing the ceramic powder after the ceramic diffusion reaction kettle, and inducing the ceramic, and igniting the ceramic buried reaction kettle for inducing the ceramic to obtain a ceramic.
Examples 2 to 4
The process steps of examples 2-4 are identical to example 1, except that: the amounts of tert-butanol used were 116.4g, 149.7g and 199.6g, respectively, which correspond to a theoretical solids content of 30 vol.%, 25 vol.% and 20 vol.%, respectively.
Example 5
Respectively weighing 40g of silicon powder, 60g of silicon nitride diluent, 2g of yttrium oxide powder, 119.5g of tertiary butanol (obtained by calculating 30 vol.% of theoretical solid content), 2g of PVB binder and 200g of silicon nitride grinding balls, adding the above six materials into a ball-milling tank made of polytetrafluoroethylene material, carrying out ball milling and uniformly mixing for 2 hours on a planetary ball mill at the speed of 300r/min to prepare uniform and stable mixture slurry, adding 1-2 drops of 1-octanol into the prepared slurry, placing the slurry in a vacuum bubble removal machine for bubble removal for 20 minutes, pouring the slurry after bubble removal into a polytetrafluoroethylene mold with the bottom surface being 50cm and × 50cm of an aluminum plate, surrounding the periphery and the top of the mold with a cotton heat insulation plate, placing the mold in a freezing box at-80 ℃ for freezing and curing for 12 hours, taking out the completely cured sample from the mold, placing the sample in a vacuum drying machine for drying for 24 hours, controlling the air pressure of the drying box within 1000pa, placing the dried blank in a muffle furnace for binder removal, raising the temperature to 600 hours, cooling the sample to 600 ℃, placing the ceramic precursor in a ceramic embedding furnace, introducing the ceramic precursor and cooling the ceramic powder together to obtain a ceramic embedding reaction kettle after ignition and inducing the ceramic diffusion reaction, and igniting the ceramic precursor after the ceramic precursor for 30-spreading, and inducing the ceramic diffusion reaction kettle under the ceramic.
Example 6
The method steps in this example 6 are the same as those in example 5, except that: the freezing temperature was-50 ℃.
Example 7
The process steps in this example 7 are the same as those in example 1, except that: the amount of tert-butanol used was 449.1g, which corresponds to a theoretical solids content of 10 vol.%.
Example 8
The process steps in this example 8 are the same as in example 1, except that: the amount of tert-butanol used was 49.9g, which corresponds to a theoretical solids content of 50 vol.%.
Comparative example 1
The procedure of comparative example 1 is the same as that of example 1 except that: the amount of tert-butanol used was 948.1g, which corresponds to a theoretical solids content of 5 vol.%.
Comparative example 2
The procedure of comparative example 2 is the same as that of example 1 except that: the using amount of the tertiary butanol is 33.3g, the corresponding theoretical solid content is 60 vol.%, fluid slurry cannot be formed after ball milling, and freezing solidification under a single-phase cold source is not facilitated.
The theoretical porosity, measured porosity, shrinkage, compressive strength in the axial direction, compressive strength in the transverse direction, and average pore diameter data of the porous silicon nitride ceramics of the oriented pore structure obtained in the above examples and comparative examples are shown in table 1.
Table 1:
as can be seen from Table 1, the amount of tert-butanol added mainly affects the porosity and the average pore size, and the higher the tert-butanol content, the higher the porosity and the larger the pore size.
It should be understood that the above-mentioned embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention, and it will be obvious to those skilled in the art that other variations or modifications may be made on the basis of the above description, and all embodiments may not be exhaustive, and all obvious variations or modifications may be included within the scope of the present invention.
Claims (10)
1. A preparation method of porous silicon nitride ceramics with a directional pore structure is characterized by comprising the following steps:
(1) dispersing silicon powder, silicon nitride powder, a sintering aid and a binder in tert-butyl alcohol to obtain ceramic slurry;
(2) pouring the obtained ceramic slurry into a mold, freezing and solidifying under a one-way cold source, and drying and removing gel to obtain a ceramic precursor;
(3) and igniting the ceramic precursor embedded by adopting embedded powder consisting of silicon powder and silicon nitride powder in a nitrogen atmosphere to initiate self-propagating synthesis to obtain the porous silicon nitride ceramic with the directional pore structure.
2. The preparation method according to claim 1, wherein in the step (1), the particle size of the silicon powder is 0.5-5 μm, the particle size of the silicon nitride powder is 0.5-5 μm, and the α phase content is 90-99 wt%.
3. The preparation method according to claim 1 or 2, wherein in the step (1), the mass ratio of the silicon powder to the silicon nitride powder is 6-3.5: 4 to 6.5.
4. The production method according to any one of claims 1 to 3, wherein in the step (1), the sintering aid is selected from at least one of yttrium oxide, ytterbium oxide, cerium oxide, and lanthanum oxide; the addition amount of the sintering aid is 1-8 wt% of the total mass of the silicon powder and the silicon nitride powder, and preferably 2-5 wt%.
5. The preparation method according to any one of claims 1 to 4, wherein in the step (1), the binder is selected from polyvinyl butyral PVB, and is added in an amount of 1 to 5wt% of the total mass of the silicon powder and the silicon nitride powder.
6. The method according to any one of claims 1 to 5, wherein in the step (1), the solid content of the ceramic slurry is calculated according to the solid content of silicon powder after the silicon powder is completely nitrided to form silicon nitride, and the tertiary butanol is used in an amount such that the solid content of the ceramic slurry is 10 to 50vol.%, preferably 20 to 40 vol.%.
7. The production method according to any one of claims 1 to 6, wherein in the step (2), before the ceramic slurry is poured into the mold, a defoaming agent is added and vacuum defoaming is performed, the defoaming agent being 1-octanol; the freezing temperature of the one-way cold source is-18 to-80 ℃; the temperature of the rubber discharge is 500-600 ℃, and the time is 1-3 hours.
8. The production method according to any one of claims 1 to 7, wherein in the step (3), the pressure of the nitrogen atmosphere is 2 to 20MPa, preferably 3 to 15 MPa.
9. The oriented pore structure porous silicon nitride ceramic prepared according to the preparation method of any one of claims 1 to 8, wherein the oriented pore structure porous silicon nitride ceramic has a microstructure of long rod-shaped mutually overlapped grains, the pore structure is oriented pores, the pore diameter is 10-200 μm, and the phase composition is β -Si3N4。
10. The oriented porous silicon nitride ceramic according to claim 9, wherein the oriented porous silicon nitride ceramic has a total porosity of 50 to 90%, an axial compressive strength of 3 to 120MPa, and a transverse compressive strength of 1 to 60 MPa.
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