CN113061040B - Preparation method of porous boron nitride ceramic - Google Patents
Preparation method of porous boron nitride ceramic Download PDFInfo
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Abstract
The invention belongs to the technical field of porous ceramic materials, and provides a preparation method of porous boron nitride ceramics, which comprises the steps of dispersing a raw material solution, taking high-purity silicon nitride powder or a polysilazane precursor as a pore-forming agent, and dispersing the pore-forming agent in the raw material solution in liquid borazine in a mass percentage of 10-20 wt.% relative to the liquid borazine; designing a stainless steel mold or a polytetrafluoroethylene forming mold, and pouring the raw material mixed solution into the mold; curing and demolding the mixed solution to obtain a co-cured product of borazine and a pore-forming agent; presintering the cured product in an inert atmosphere sintering furnace, heating to 1200 ℃ at a heating rate of 5 ℃/min, and preserving heat until the mineralization of borazine ceramic is completed; heating to 1850-2000 ℃ in a pyrolysis furnace at the heating rate of 10-20 ℃/min, and preserving the temperature until the pore-forming agent is completely burnt out, thus obtaining the porous boron nitride ceramic. The method has the characteristics of simple and controllable process and strong operability, and can be used for preparing boron nitride porous ceramic materials and components with complex shapes.
Description
Technical Field
The invention belongs to the technical field of porous ceramic materials, and particularly relates to a preparation method of porous boron nitride ceramic.
Background
The porous boron nitride ceramic has the characteristics of high temperature resistance, erosion resistance and dielectric stability of the boron nitride ceramic, high porosity, low density and high specific surface area of a porous material, and has wide application prospects in the aspects of aerospace, special smelting, high-temperature wave transmission, high-temperature filtration, high-temperature catalyst carriers and the like.
The preparation method of the porous boron nitride ceramic mainly comprises a template method, a foaming method, an element replacement method, a pore-forming agent burning method and the like. Wherein, the microscopic appearance of the product obtained by the template method is relatively uniform, but the process is complex and the cost is high; the foaming method is simple to operate, but toxic gas is easily generated in the preparation process, so that the environment is not protected; the element substitution method needs a series of chemical reactions, and the product has more impurities. The principle of the pore-forming agent burning-out method is that organic matter or inorganic powder with lower decomposition temperature is added into a boron nitride raw material as a second phase, porous boron nitride ceramics can be obtained after the pore-forming agent is burnt out at high temperature, and the boron nitride ceramics is sintered at high temperature to form a porous ceramic network; however, boron nitride is a typical covalent bond compound, has poor sintering performance, and usually requires active addition of a sintering aid, which causes residual impurity phases or pore-forming agent residual elements in the porous ceramic, thereby affecting the purity and performance of the porous ceramic. Therefore, there is a need to further explore a new porous boron nitride ceramic with high purity, controllable porosity and pore morphology, stable structural components and simple preparation method and a preparation method thereof.
Disclosure of Invention
The invention aims to solve the technical problems that the existing preparation method of porous boron nitride ceramics is complex and the ceramics contains impurity phases, and provides the following technical scheme for solving the technical problems:
the invention provides a preparation method of porous boron nitride ceramic, which comprises the following steps:
step 3, curing the mixed solution, namely transferring a mold containing the raw material mixed solution into an autoclave, filling 2-5 MPa argon as protective gas, heating to 300 ℃ at a heating speed of 3-5 ℃/min, and preserving heat until the solution in the mold is cured in situ;
step 4, demolding the cured product, naturally cooling the autoclave to room temperature, taking out the mold, and demolding to obtain a co-cured product of borazine and the pore-forming agent;
step 5, presintering the cured product in an inert atmosphere sintering furnace, raising the temperature to 1200 ℃ at the temperature rise speed of 5 ℃/min, and preserving the temperature until the mineralization of the borazine ceramic is completed;
and 6, burning off the pore-forming agent at high temperature, heating to 1850-2000 ℃ in a pyrolysis furnace at a heating rate of 10-20 ℃/min, and preserving the temperature until the pore-forming agent is completely burnt off, thus obtaining the porous boron nitride ceramic.
Further, in the raw material solution in the step 1, when the pore-forming agent is silicon nitride powder, the particle size of the silicon nitride powder is 0.5-100 μm;
further, in the raw material solution in step 1, when the polysilazane precursor is used as the pore-forming agent, the physical state of the polysilazane precursor is liquid.
Furthermore, the density of the porous boron nitride ceramic prepared by the invention is between 0.42 and 1.35g/cm3The porosity is between 35% and 85%, the pore size is between 0.5 mu m and 500 mu m, the dielectric constant is between 1.34 and 2.56, the loss tangent is between 0.0015 and 0.005, and the content of boron nitride is more than 98%.
Compared with the prior art, the invention has the following effective benefits:
1. the preparation method of the porous boron nitride ceramic has the characteristics of simple and controllable process and strong operability, and can be used for preparing the boron nitride porous ceramic material and the member with complex shapes. Borazine has good fluidity, is beneficial to filling complex mold space, can be shaped after in-situ curing, can effectively avoid cracking deformation in the subsequent pore-forming agent burning-off process, has good machinability of porous boron nitride ceramics, and can be ground and finely processed according to the actual size requirement.
2. The preparation method of the porous boron nitride ceramic has the characteristic of flexible and adjustable porosity, and the porosity can be adjusted within the range of 35-80% by adjusting the content of the pore-forming agent. In addition, when different pore-forming agent types are adopted, the microstructure and the multi-level pore structure of the boron nitride continuous network can be flexibly adjusted, so that the boron nitride continuous network can be used according to different use environments.
3. The porous boron nitride ceramic prepared by the method has the characteristics of high purity and high stability of structure, a porous network framework is formed by means of the self-crosslinking characteristic and the in-situ curing characteristic of borazine, a sintering aid is not required to be added, the pore-forming agent can be completely burnt out at high temperature, and the purity of the prepared porous ceramic is high.
4. The porous boron nitride ceramic prepared by the invention has wide application. The porous boron nitride ceramic has the characteristics of high porosity, adjustable porosity and nonwetting molten metal, and can be used for preparing a metal porous filtering material; the porous boron nitride ceramic has low density, high purity and small dielectric constant and loss tangent, and can be used as an electromagnetic wave-transmitting material of an aircraft; the porous boron nitride ceramic has the characteristics of high temperature resistance, corrosion resistance and large specific surface area, and can be used for preparing filters for high-temperature gas purification or automobile exhaust treatment and the like.
Drawings
FIG. 1 is a flow chart of a specific process for preparing a porous boron nitride ceramic according to the present invention;
FIG. 2 is a topographical view of the porous boron nitride ceramic prepared in example 1, at 30 times magnification;
FIG. 3 is a topographical view of the porous boron nitride ceramic prepared in example 1, at 100 times magnification;
FIG. 4 is a graphical representation of the morphology of the porous boron nitride ceramic prepared in example 1, at a magnification of 15000 times;
FIG. 5 is a spectrum of the porous boron nitride ceramic prepared in example 1;
FIG. 6 is an XRD pattern of a ceramic product pre-sintered at 1200 ℃ and sintered at high temperature of 2000 ℃ after the raw material mixed solution of example 1 is solidified;
FIG. 7 is a topographical view of the porous boron nitride ceramic prepared in example 2, at 2000 times magnification;
FIG. 8 is a topographical view of the porous boron nitride ceramic prepared in example 2, at 6000 times magnification;
FIG. 9 is a topographical view of the porous boron nitride ceramic prepared in example 2, at 45000 times magnification;
FIG. 10 is an XRD pattern of a ceramic product pre-sintered at 1200 ℃ and high-temperature sintered at 1850 ℃ after the raw material mixed solution of example 2 was solidified.
DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION
The invention is explained and illustrated in detail below with reference to the figures and examples.
The invention provides a preparation method of porous boron nitride ceramics, and figure 1 is a specific process flow chart of the preparation method of the porous boron nitride ceramics, which comprises the following steps:
step 3, curing the mixed solution, namely transferring a mold containing the raw material mixed solution into an autoclave, filling 2-5 MPa argon as protective gas, heating to 300 ℃ at a heating rate of 3-5 ℃/min, and preserving heat for 6-8 hours to enable the solution in the mold to be cured in situ;
step 4, demolding the cured product, naturally cooling the autoclave to room temperature, taking out the mold, and demolding to obtain a co-cured product of borazine and the pore-forming agent;
step 5, presintering the cured product in an inert atmosphere sintering furnace, raising the temperature to 1200 ℃ at the temperature rise speed of 5 ℃/min, and preserving the temperature for 2 hours to finish the mineralization of borazine ceramic;
and 6, burning off the pore-forming agent at high temperature, heating to 1850-2000 ℃ in a pyrolysis furnace at a heating rate of 10-20 ℃/min, preserving the temperature for 1-2 hours, and completely burning off the pore-forming agent to obtain the porous boron nitride ceramic.
Further, in the raw material solution in the step 1, the mass percentage of the pore-forming agent relative to the liquid borazine is 10-50 wt.%;
further, in the raw material solution of step 1, when the pore-forming agent is silicon nitride powder, the particle size of the silicon nitride powder is 0.5 μm to 100 μm;
further, in the raw material solution in step 1, when the polysilazane precursor is used as the pore-forming agent, the physical state of the polysilazane precursor is liquid.
Furthermore, the density of the porous boron nitride ceramic prepared by the invention is between 0.42 and 1.35g/cm3The porosity is between 35% and 80%, the pore size is between 0.5 mu m and 500 mu m, the dielectric constant is between 1.34 and 2.56, the loss tangent is between 0.0015 and 0.005, and the content of boron nitride is more than 98%.
In the method for preparing the porous boron nitride ceramic, two main pore generation modes are adopted in the porous ceramic, namely small molecules such as hydrogen and the like are released in the processes of borazine solidification, mineralization and high-temperature cracking, volume shrinkage and networking connection are generated during cross-linking and sintering, pores generated in the process are large, the relationship between the porosity and the temperature rise rate, the solidification temperature and the pressure of protective inert gas in the solidification process is the largest, and the adjustable range of the total porosity is small. And secondly, pores generated by the escape of the pore-forming agent due to high-temperature burning-out can be effectively adjusted according to the change of the mass percent of the pore-forming agent, and the multi-layer pore structure of the porous boron nitride ceramic can be adjusted. Specifically, when silicon nitride powder is used as a pore-forming agent, the silicon nitride powder is uniformly dispersed in solid polyborazine after the mixed solution is solidified, and when the pore-forming agent is burnt out above the decomposition temperature of the silicon nitride ceramic, the silicon nitride powder is completely decomposed, decomposed gas escapes and pores are left, and the size of the pores is equivalent to the particle diameter of the powder; when the liquid polysilazane precursor is used as a pore-forming agent, in the process of raw material solution dispersion and in-situ curing, the dispersion between the liquid polysilazane and the liquid borazine is better than that of silicon nitride powder, the liquid polysilazane and the liquid borazine form a network cross-linking structure after being cured, small molecules are removed from the liquid polysilazane and the silicon nitride powder in the process of pre-sintering, a silicon nitride ceramic phase is mainly generated when the polysilazane is subjected to high-temperature cracking, and the silicon nitride ceramic phase can be completely decomposed and escaped at the temperature higher than 1850 ℃. Because the silicon nitride ceramic formed by cracking and converting polysilazane has more uniform distribution in borazine and smaller particle size, even reaches the level of atomic group and crystal grain, the pores generated after cracking are obviously smaller than those generated by adopting silicon nitride powder as a pore-forming agent, and the pores of 0.5 mu m or smaller are generated in the boron nitride ceramic, thereby embodying the adjustment and control of a multi-layer pore structure.
The preparation method has the advantages of few steps, simple and controllable process flow, good operability, high yield and stable performance of finished products, and fully utilizes the fluidity of the liquid precursor to realize the preparation of porous ceramics with different shapes.
Two examples of specific implementations of the invention are given below.
Example 1
In this example, porous boron nitride ceramics with a size of 50mm × 50mm × 30mm and silicon nitride powder as a pore-forming agent were prepared. The preparation process comprises the following steps:
(1) dispersing a raw material solution, namely dispersing a high-purity silicon nitride powder serving as a pore-forming agent, wherein the particle size of the silicon nitride powder is 1 mu m, dispersing the pore-forming agent in a liquid borazine precursor according to the mass percentage of 15 wt%, and dispersing in a magnetic stirring manner to obtain a mixed solution;
(2) pouring the mixed solution, namely preparing a container with the internal dimension of 70mm multiplied by 50mm by adopting a polytetrafluoroethylene sheet, and pouring the mixed solution into a mold;
(3) curing the mixed solution, transferring the combined mold containing the mixed solution into an autoclave, introducing 3MPa argon as protective gas, heating to 300 ℃ at the heating rate of 5 ℃/min, and preserving heat for 6 hours to cure the solution in the mold;
(4) demoulding the cured product, naturally cooling the autoclave to room temperature, taking out the mould, and demoulding to obtain a co-cured product of borazine and the pore-forming agent;
(5) presintering the cured product, presintering the cured product in an inert atmosphere sintering furnace, heating to 1200 ℃ at the heating rate of 5 ℃/min, and preserving heat for 2h to finish the mineralization of borazine ceramic;
(6) and (3) burning off the pore-forming agent at high temperature, heating to 2000 ℃ at a heating rate of 10 ℃/min in a pyrolysis furnace, preserving the temperature for 1h, and completely burning off the pore-forming agent to obtain the porous boron nitride ceramic.
The final dimensions of the porous boron nitride ceramic prepared in this example were 55 mm. times.55 mm. times.33 mm, the porosity of the porous ceramic was 71%, and the density was 1.47g/cm3The boron nitride content was about 99%, the boron nitride content was 99.2%, the dielectric constant was 1.56, and the loss tangent was 0.004.
Fig. 2 and 3 are morphology diagrams of the porous boron nitride ceramic prepared by the method of the invention, and it can be seen that the prepared boron nitride ceramic is mainly based on large through holes, and the pores with the maximum size of up to-500 μm mainly come from escape of small molecule gas generated by cracking borazine and escape of decomposed gas generated by pore-forming agent. The decomposition temperature of the silicon nitride powder is 1850 ℃ or lower, and the silicon nitride powder is completely decomposed at a high temperature of 2000 ℃, thereby leaving pores.
FIG. 4 is a microscopic morphology of the porous boron nitride ceramic prepared by the method of the present invention, which shows that the porous network frame and the boron nitride ceramic at the pore wall present a compact lamellar structure, which is a typical structure of h-BN, and indicates that the h-BN porous network is effectively formed by high temperature sintering at 2000 ℃. Fig. 5 is an energy spectrum of the porous boron nitride ceramic prepared in this example, and it can be seen that the main elements of the porous boron nitride ceramic are boron and nitrogen. FIG. 6 is an XRD pattern of a ceramic product pre-sintered at 1200 ℃ and sintered at 2000 ℃ after the raw material mixed solution is solidified, and diffraction peaks of boron nitride ceramic and silicon nitride ceramic which are not completely crystallized appear in the product after the pre-sintering at 1200 ℃, which are respectively attributed to the ceramic converted by cracking of borazine and the diffraction peaks attributed to silicon nitride powder with good crystallinity; after sintering at 2000 ℃, only a diffraction peak belonging to h-BN with good crystallinity exists, which shows that the pore-forming agent silicon nitride powder is completely decomposed and escaped, and the good crystallinity is also beneficial to improving the temperature resistance and the structural stability of the porous ceramic.
Example 2
In this example, porous boron nitride ceramics with a size of Φ 50mm × 40mm and a liquid polysilazane precursor as a pore-forming agent were prepared. The preparation process comprises the following steps:
(1) dispersing a raw material solution, namely dispersing a pore-forming agent in a liquid borazine precursor according to the mass percentage of 20 wt.% by taking a liquid polysilazane precursor as the pore-forming agent, and dispersing in a magnetic stirring manner to obtain a mixed solution;
(2) pouring the mixed solution, namely preparing a container with the internal dimension of phi 60mm multiplied by 60mm by adopting a polytetrafluoroethylene sheet, and pouring the mixed solution into a mold;
(3) curing the mixed solution, transferring the combined mold containing the mixed solution into an autoclave, introducing 5MPa argon as protective gas, heating to 300 ℃ at the heating rate of 3 ℃/min, and preserving heat for 8 hours to cure the solution in the mold;
(4) demoulding the cured product, naturally cooling the autoclave to room temperature, taking out and demoulding to obtain a co-cured product of borazine and a pore-forming agent;
(5) presintering the cured product, presintering the cured product in an inert atmosphere sintering furnace, heating to 1200 ℃ at the heating rate of 5 ℃/min, and preserving heat for 2h to finish the mineralization of borazine ceramic;
(6) and (3) burning off the pore-forming agent at high temperature, heating to 2000 ℃ at a heating rate of 10 ℃/min in a pyrolysis furnace, preserving the temperature for 1h, and completely burning off the pore-forming agent to obtain the porous boron nitride ceramic.
The final physical dimension of the porous boron nitride ceramic prepared in this example was phi 52mm × 45mm, the porosity of the porous ceramic was 78%, and the density was 1.52g/cm3The boron nitride content was about 98.5%, the dielectric constant was 1.60, and the loss tangent was 0.005.
FIGS. 7, 8 and 9 are the microscopic morphology diagrams of the porous boron nitride ceramics prepared by the method of the present invention, and it can be seen that the prepared boron nitride ceramics also contains a large number of fine interconnected pores, and the minimum pore size is about 0.5 μm, which is the pore left by the complete decomposition of polysilazane decomposition product when the pore-forming agent is removed at 1850 ℃. Similarly, boron nitride ceramics at the pore wall have a sheet structure, which is a typical structure of h-BN. FIG. 10 is an XRD pattern of a ceramic product obtained by pre-sintering at 1200 ℃ and sintering at 1850 ℃ after the raw material mixed solution is solidified, and it can be seen that diffraction peaks of boron nitride ceramic and silicon nitride ceramic which are not completely crystallized appear in the product after the pre-sintering at 1200 ℃ are respectively assigned to the ceramic and silicon nitride powder subjected to the pyrolysis conversion of borazine; after sintering at 1850 ℃, the existence of a diffraction peak attributed to h-BN with good crystallinity shows that the pore-forming agent silicon nitride powder is completely decomposed and escaped, and the good crystallinity is also beneficial to improving the structural stability of the porous ceramic.
In conclusion, the material composition, the preparation process and the ceramic performance of the porous boron nitride ceramic have obvious design characteristics, and the porosity, the pore morphology and the dielectric property of the prepared ceramic can be controlled and adjusted through the components and the content of the pore-forming agent, so that the performance of the porous boron nitride ceramic can be designed according to different application environments and performance requirements, and the porous boron nitride ceramic can be widely applied to the fields of high-temperature crucibles, special smelting, high-temperature filtration, high-temperature catalysis and the like.
Claims (4)
1. A preparation method of porous boron nitride ceramics is characterized by comprising the following steps:
dispersing a raw material solution, namely dispersing the high-purity silicon nitride powder or a polysilazane precursor serving as a pore-forming agent in the raw material solution, wherein the pore-forming agent is dispersed in liquid borazine in a mass percent of 10-20 wt.% relative to the liquid borazine by adopting a magnetic stirring mode to obtain a mixed solution of the raw materials;
step 2, pouring the mixed solution, designing a stainless steel mold or a polytetrafluoroethylene forming mold according to the appearance requirements of the required porous ceramic and the required member, and pouring the raw material mixed solution into the mold;
step 3, curing the mixed solution, namely transferring a mold containing the raw material mixed solution into an autoclave, filling 2-5 MPa argon as protective gas, heating to 300 ℃ at a heating speed of 3-5 ℃/min, and preserving heat until the solution in the mold is cured in situ;
step 4, demolding the cured product, naturally cooling the autoclave to room temperature, taking out the mold, and demolding to obtain a co-cured product of borazine and the pore-forming agent;
step 5, presintering the cured product in an inert atmosphere sintering furnace, raising the temperature to 1200 ℃ at the temperature rise speed of 5 ℃/min, and preserving the temperature until the mineralization of the borazine ceramic is completed;
and 6, burning off the pore-forming agent at high temperature, heating to 1850-2000 ℃ in a pyrolysis furnace at a heating rate of 10-20 ℃/min, and preserving the temperature until the pore-forming agent is completely burnt off, thus obtaining the porous boron nitride ceramic.
2. The method according to claim 1, wherein in the step 1, when the pore-forming agent in the raw material solution is silicon nitride powder, the particle size of the silicon nitride powder is 0.5 μm to 100 μm.
3. The method according to claim 1, wherein in step 1, when the polysilazane precursor is used as the pore-forming agent in the raw material solution, the physical state of the polysilazane precursor is liquid.
4. The method according to any one of claims 1 to 3, wherein the porous ceramic has a density of 0.42 to 1.35g/cm3The porosity is between 35% and 85%, the pore size is between 0.5 mu m and 500 mu m, the dielectric constant is between 1.34 and 2.56, the loss tangent is between 0.0015 and 0.005, and the content of boron nitride is more than 98%.
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