Scientists once predicted that nanomaterials will become "the most promising materials" in the 21st century, and included nanotechnology research as one of the 11 key areas in the first 10 years of the 21st century, and these are all related to "nano composite ceramics". The outstanding performance is inseparable.
As early as the late 1990s, a research team led by a professor named Niihara reported some exciting experimental results about nanocomposite ceramics, such as Al2O3-SiC (5% by volume) intragranular nanocomposite The room temperature strength of ceramics is 3 to 4 times that of single-component Al2O3 ceramics, and the strength is 1500MPa at 1100°C. These have aroused great interest from material researchers. From then until now, the research of nanocomposite ceramics has been intensified.
Overview of Nanocomposite Ceramics
Nano-composite ceramics refer to composite materials obtained by effectively dispersing and recombining so that the heterogeneous phase (second phase) nanoparticles are uniformly dispersed and retained in the ceramic matrix. Compared with single-component ceramics, the performance of nanocomposite ceramics has been greatly improved, and the magnitude is far beyond that of general micron-level composite ceramics.
This is directly related to the change of its microstructure, such as: the type, quantity, grain size and size distribution of nanoparticles, morphology, grain boundaries, intermediate phases, intracrystalline defects, intercrystalline purity, intercrystalline residuals Stress, etc., have a certain relationship with the improvement of performance.
The main purpose of preparing nanocomposite ceramics is to give full play to the high hardness, high temperature resistance and corrosion resistance of ceramics and improve their brittleness, which are used in very important fields such as high temperature gas turbines and aerospace components. So how are nanocomposite ceramics prepared?
Overview of preparation methods of nanocomposite ceramics
Various new nanocomposite ceramic material synthesis technologies developed at present include: mechanical mixing dispersion-forming-sintering method, liquid phase dispersion-forming-sintering method, in-situ generation-forming-sintering method, composite powder-forming -Sintering method, direct molding-sintering method, surface modification-forming-sintering method, etc., these preparation technologies have their own characteristics, and the scope of application is different. At the same time, they all have their own limitations. Most of these technologies are due to expensive equipment and process. It is complicated and difficult to control and has been in the laboratory research stage for a long time.
1. Mechanical mixing dispersion-forming-sintering method
The main process of the mechanical mixing and dispersion method is to mix the matrix powder and the nano powder, ball mill, and then sinter. The disadvantage is that the ball milling itself cannot completely destroy the agglomeration between the nanoparticles, and can not guarantee the uniform dispersion of the two-phase composition, so that the particles agglomerate and settle after the ball milling, causing further unevenness.
2. Liquid phase dispersion-forming-sintering method
The main process of the liquid phase dispersion method is to disperse the obtained nano powder in a solution containing matrix components, and adjust the process parameters to freeze and gel the system without crystallization, agglomeration, and sedimentation. After heat treatment The composite powder is obtained. The nanocomposite ceramics prepared by this method have more precise microstructures and improved mechanical properties than samples prepared by mechanical ball milling.
2. In-situ generation-forming-sintering method
As a breakthrough composite technology, in-situ generation technology is generally valued by scholars at home and abroad. The principle is: select the appropriate reactant (gas, liquid or solid phase) according to the requirements of material design, and use the physical and chemical reaction between the substrates at the appropriate temperature to generate a uniformly distributed second phase (or Called enhanced phase).
Because the in-situ generation technology can basically overcome a series of problems that usually occur in other processes, such as overcoming the poor infiltration of the matrix and the second phase or the reinforcement, the interface reaction produces a brittle layer, and the uneven distribution of the second phase or the reinforcement phase, especially The tiny (sub-micron and nano-scale) second phases or reinforcement phases are extremely difficult to recombine, so they have great potential in the development of new metal-based nanocomposites.
3. Composite powder-molding-sintering method
The composite powder method is to directly prepare a composite powder in which the matrix and the dispersed phase are uniformly dispersed through chemical and physical processes, and then form and sinter. The preparation methods include CVD method, precursor conversion method, laser synthesis method and so on. Using this method, the composite powder already contains all the required nanophases, and the nanophases have been evenly dispersed in the composite powder, and there is no problem of nanophase dispersion and agglomeration. Using this method to prepare nanocomposite materials can obtain high strength.
4. Direct molding-sintering method
The re-aggregation and abnormal growth of nanoparticles in the process of drying and densification are one of the difficulties in preparing nanomaterials. The direct molding method is to make a uniformly dispersed slurry with high solid content, and then directly injection molding. Under the action of initiator or biological enzyme, the system is quickly gelled to obtain a green body, which avoids the drying of the particles. Re-agglomeration can achieve proportional shrinkage during sintering and can be made into parts with complex shapes, which is an important advancement in ceramic molding methods.
5. Surface modification-forming-sintering method
This method is to chemically or physically wrap the surface of micron or nano powders with a layer of oxide or graft small molecular chains or polymer chains to change the acidity, isoelectric point and electrokinetic potential of the surface of the colloidal particles, and prevent the nanoparticles The agglomeration improves the dispersion effect. The coating modification of the particle surface is mainly through precipitation reaction coating and sol-gel reaction coating.
Limitations of partial preparation technology
The powder prepared by the mechanical mixing dispersion method has the problems of wide particle size distribution and uneven powder distribution; the dispersant that plays a key role in the liquid dispersion method is currently few serialized products, and the research is not systematic enough; surface modification The method first disperses the nanophase before the wet ball milling of the ceramic particles, which improves the material performance compared with the liquid phase dispersion method. However, the nanophase dispersion problem still exists in the surface modification and subsequent ball milling process; the direct molding method is low in cost. It is an ideal method for mass production of high-performance ceramics that can form complex parts of various shapes. However, preparing ceramic slurry with high solid content is one of its difficulties.
Sintering technology is particularly important
The emergence of nano-composite ceramics provides a feasible way to solve the brittleness of ceramics. Because nano-particles can inhibit the growth of matrix particles, the structure is refined, thereby improving the mechanical properties of materials. But at the same time, due to the extremely large specific surface area, the nanoparticles are prone to grow up during the sintering process, thereby losing the characteristics of the nanoparticles. As a result, not only the performance of the material cannot be improved, but the performance of the material may be seriously reduced. In the sense of nanocomposite ceramics, it is particularly important to choose a suitable sintering technology.
Because the traditional sintering method requires higher temperature and longer holding time, it is difficult to maintain the nano-scale (<100nm) microstructure in the sintered nano-ceramics, thus losing the characteristics of nano-materials. Based on this, a variety of advanced sintering technologies such as hot pressing sintering, hot isostatic pressing sintering, ultra-high pressure sintering, spark plasma sintering, microwave sintering, selective laser sintering, explosion-explosion sintering and reaction sintering have been produced.
The sintering technology of nano-composite ceramic materials is designed to control grain growth by controlling the sintering temperature and sintering time. Different sintering technologies have their own advantages and disadvantages, and have certain limitations in application. Different materials may use different sintering technologies or processes. How to select sintering technology reasonably and optimize sintering process parameters needs to be improved.