6 new rapid sintering technologies for advanced ceramics

Advanced ceramic materials play a vital role in the industrial field with a series of excellent properties. However, the wide application of ceramic materials still faces many problems and challenges, among which reliability, density and strength are the main constraints. How to achieve rapid densification of materials at a lower sintering temperature and prepare ceramic blocks that are completely free of porosity, uniform structure, fine grains and strengthened grain boundaries are still the goal of ceramic materials scientists. The traditional ceramic sintering technology, including atmosphere sintering, vacuum sintering, hot pressing sintering and hot isostatic pressing sintering, is mainly to keep the ceramic powder under the action of high temperature thermal driving force for a long time, and use atomic diffusion to remove the inter-grain The process of densification of the pores. However, under high temperature conditions, while promoting the densification of the material, the diffusion of atoms will inevitably lead to the growth of crystal grains, thereby degrading the performance of the material. The holding time of several hours or even days is a great consumption of energy and is not conducive to industrial production.
 
In the innovation of advanced ceramic preparation technology and manufacturing equipment, the innovation of ceramic sintering equipment and sintering technology is one of the most critical factors to further improve the performance of advanced ceramic materials. In this regard, scientific researchers have successfully developed a variety of new ceramic sintering processes, which may reduce the sintering temperature and shorten the sintering time to achieve rapid densification; or can improve the properties of materials.
 
1 Self-propagating high temperature sintering (SHS)
 
Self-propagation High temperature Synthesis (SHS), also known as Combustion Synthesis (Combustion Synthesis abbreviation CS) is a material preparation technology that quickly emerged in the 1980s. A material sintering process proposed by the former Soviet Union scientist Merzhanov. This method is based on the principle of exothermic chemical reaction. It uses external energy to induce local chemical reactions to form a chemical reaction front (combustion wave). After that, the chemical reaction continues under the support of its own heat release. As the combustion wave advances, The combustion spreads to the entire system, synthesizing the required materials. The method has simple equipment and process, rapid reaction, high product purity and low energy consumption. It is suitable for the synthesis of non-stoichiometric compounds, intermediate products and metastable equivalents.
 
Since the 1980s, self-propagating sintering technology has developed rapidly, and has been successfully applied to industrial production, combined with technologies in many other fields to form a series of related technologies, such as SHS powder synthesis technology, SHS sintering technology, and SHS compacting Chemical technology, SHS metallurgical technology, etc.
 
SHS densification technology means that the product in the SHS process is in a hot thermoplastic state with the help of external loads, which can be static or dynamic or even explosive shock loads to achieve densification, and sometimes high-pressure inert atmosphere is used to promote densification. This is because the product usually obtained by self-propagating high-temperature synthesis is in a loose state, generally containing 40% to 50% of residual pores.
 
SHS densification processes that are currently studied more include: SHS-quasi-isostatic pressing method (SHS-PIP); thermal explosion-pressurization method; high pressure self-combustion sintering method (HPCS); gas pressure combustion sintering method (GPCS); SHS- Explosive shock loading method (SHS/DC); SHS-centrifugal densification, etc. Among them, the method is the action of applied mechanical pressure, the method is the action of centrifugal force, and the method is the action of gas pressure.
 
It can be used to make protective coatings, abrasive pastes, polishing powders, tools, heating components, shape memory alloys, ceramic-metal welding, etc. However, the SHS process research needs to be further deepened, and the research on the process of product densification and one-step net molding products needs to be strengthened. Give full play to its high-efficiency and energy-saving advantages, making it move from the experimental stage to industrialized production.
 
2 Microwave sintering
 
Microwave sintering uses the dielectric loss of the ceramic material in the microwave electromagnetic field to bring the material to the sintering temperature to achieve the sintering and densification of the ceramic. During microwave sintering, the material absorbs the microwave and converts it into the kinetic energy and potential energy of the internal molecules of the material, so that the material is heated uniformly, the internal temperature gradient is small, and the heating and sintering speed is fast. It can realize rapid sintering at low temperature and significantly improve the mechanical properties of ceramic materials. In addition, microwave sintering does not require a heat source, which is highly efficient and energy-saving. High production efficiency, low cost per piece. It has broad application prospects in the field of ceramic material preparation, and provides a new way to prepare sub-meter or even micron ceramic materials.
 
Microwave sintering technology was proposed in the mid-1960s. Since the 1970s, microwave sintering technology has been systematically studied at home and abroad, including sintering mechanism, device optimization, dielectric parameters, and sintering process. In the late 1990s, microwave sintering entered the industrialization stage. Microwave sintering technology is used to produce various materials such as originals of optical fiber materials, ferrites, superconducting materials, lithium hydride, and nanomaterials. IndexTool of Canada uses microwave sintering to manufacture SiaNa tools. The United States, Canada and other countries use microwave sintering to produce spark plug porcelain, ZrO2, SiN4, SiC, Al2O3, TiC, etc. in batches.
 
However, microwave sintering technology has not yet reached a mature industrialization level. In-depth research is needed for the determination of basic parameters such as dielectric properties and database establishment, sintering compaction mechanism, microstructure evolution process, furnace structure and heat preservation device, etc., to promote ceramic materials Microwave sintering is developing towards industrialization.
 
3 Spark plasma sintering (SPS)
 
SPS technology is a new type of rapid sintering technology that has received extensive attention and research from the academic community. Figure 3 shows a schematic diagram of its working principle. The SPS technology pioneered the introduction of DC pulse current into the sintering process, and the indenter also acts as a carrier through which the current passes while applying pressure to the material. Unlike traditional sintering technology, which usually uses heating element radiant heating, SPS technology uses the thermal effect of a large current through the mold or conductive sample to heat the material. For insulating samples, graphite with good conductivity is usually used as the mold material, and the resistance heat of the mold is used to quickly heat up the sample; for conductive samples, an insulating mold can be used to heat the sample directly through the current. The heating rate can reach 1000 ℃/min. When the sample temperature reaches the set value, the sintering can be completed after a short time of heat preservation.
 
SPS technology has outstanding advantages such as low sintering temperature, short holding time, fast heating rate, adjustable sintering pressure, and multi-field coupling (electricity-force-heat). In addition to common ceramics such as Al2O3 and ZrO2, SPS technology can also be used for the preparation of many refractory materials, such as ultra-high temperature ceramics such as ZrB2, HfB2, ZrC, TiN, and refractory metals such as W, Re, Ta, Mo and their alloys.
 
By using a specially designed mold such as a ladder to change the current density flowing through the mold, the temperature gradient can be artificially created in the sample, so the SPS technology can also be used to prepare functionally graded materials. In addition, functional materials such as nanocrystalline transparent ceramics and dielectric ceramics can also be sintered using SPS technology.
 
4 Flash burn (FS)
 
The FS technology was first reported by Cologna et al. of the University of Colorado in 2010. It originated from the study of field-assisted sintering technology (FAST). Figure 4a is a schematic diagram of a typical FS device. The ceramic green body to be sintered is made into a "bone shape". Both ends are suspended in the modified furnace body by platinum wires, and a certain DC or AC electric field is applied to the material. There is a thermocouple in the furnace body for temperature measurement, and a CCD camera at the bottom can record the sample size in real time. Taking 3YSZ as an example, the researchers found that compared with traditional sintering, if the furnace body is heated at a constant rate, applying a DC electric field strength of 20 V/cm to it can increase the sintering rate to a certain extent and reduce the sintering requirements. The furnace temperature is shown in Figure 4b. As the field strength increases, the furnace temperature required for sintering continues to decrease.
 
When the field strength is 60 V/cm, the sample will be instantly densified when the furnace temperature rises to about 1025°C; when the field strength is increased to 120 V/cm, the sintering furnace temperature can even be reduced to 850°C. This new sintering technology is called "flash sintering", which is a new type of sintering technology that achieves low-temperature and extremely rapid sintering of materials under a certain temperature and electric field. Usually, the following three phenomena will occur with FS: thermal runaway inside the material; sudden drop in the resistivity of the material itself; strong flashing phenomenon.
 
FS technology mainly involves three process parameters, namely furnace temperature (Tf), field strength (E) and current (J). Figure 4c shows the change trend diagram of various parameters in the traditional FS process. In this mode, a stable electric field is applied to the material, and the furnace temperature rises at a constant rate. When the furnace temperature is low, the material resistivity is high, and the current flowing through the material is small. As the furnace temperature increases, the resistivity of the sample decreases and the current gradually increases. This stage is called the incubation stage, and the system is voltage controlled. When the furnace temperature rises to the critical temperature, the material resistivity drops suddenly, the current rises sharply, and FS occurs. Since the field strength is still stable at this time, the system power (W = EJ) will quickly reach the power upper limit of the power supply, and the system will change from voltage control to current control. This stage is called the FS stage (flash sintering stage). When the resistivity of the material no longer increases, the field strength is stabilized again, and the sintering enters a steady stage (steady stage), that is, the holding stage of FS. After the holding stage, a complete FS process ends.
 
Compared with traditional sintering, FS has the following advantages: shortens the sintering time and reduces the furnace temperature required for sintering, inhibits grain growth, can achieve non-equilibrium sintering, simple equipment and lower cost.
 
5 Cold Sintering (CS)
 
In order to make the density of the ceramic material reach more than 95% of its theoretical density, the sintering temperature of the ceramic material needs to reach 50% to 75% of its melting temperature. Therefore, the sintering temperature of most ceramic materials is greater than 1000 ℃, making the production process of ceramic materials consume more energy, and high-temperature sintering makes ceramic materials limited in terms of material synthesis and phase stability.
 
In order to reduce the sintering densification temperature of ceramic powder, new sintering technologies such as liquid phase sintering, field-assisted sintering, and FS have been applied. However, due to solid phase diffusion and liquid phase formation, higher temperature heating of ceramic powder is still required. The above technologies do not Reduce the sintering temperature to the "low temperature range". Recently, the Andall research group of Pennsylvania State University in the United States was inspired by the hydrothermal-assisted hot pressing process and proposed a new "ceramic CS process" technology. Different from the traditional high-temperature sintering process, the ceramic CS process adds an instantaneous solvent to the powder and applies a relatively large pressure (350~500 MPa) to enhance the rearrangement and diffusion between particles, so that the ceramic powder is at a lower temperature. Temperature (120 ~ 300 ℃) and a short time to achieve sintering densification, creating the possibility for low-temperature sintering to produce high-performance structural ceramics and functional ceramics.
 
The basic process of ceramic CS technology is to add a small amount of aqueous solution to the ceramic powder to wet the particles, and the surface material of the powder is decomposed and partially dissolved in the solution, thereby generating a liquid phase at the particle-particle interface. Put the wetted powder into the mold, and heat the mold while applying a relatively large pressure. After the pressure is maintained for a period of time, a dense ceramic material can be prepared. Maria et al. observed and analyzed the preparation process of a variety of ceramic systems, and summarized the internal process of the CS process into two steps:
 
In the first stage, the mechanical pressure causes the liquid phase between the powder particles to flow, thereby triggering the rearrangement of the powder particles; in the second stage, the pressure and temperature promote the dissolution of the powder surface substances in the liquid phase, and this process The substance is diffused and transported. In the first stage, the driving force of the densification process is mainly provided by mechanical pressure. The role of the liquid phase is to promote the sliding and rearrangement of the particles, and the tip of the particles will dissolve in the liquid phase to make the particles spherical, thereby improving the particles during the compression process. The bulk density. In the second stage, the mechanical pressure and temperature will cause the solution in the system to evaporate instantaneously, and the supersaturation of the solution will increase with the sintering time. The substance diffuses in the liquid phase and precipitates on the surface of the particles away from the pressure zone. Filling in the grain boundaries or pores to densify the ceramics. At this stage, the amorphous precipitates will be pinned to the grain boundaries to inhibit the growth of crystal grains.
 
6 Oscillating pressure sintering (OPS)
 
Existing various pressure sintering technologies use static constant pressure. The introduction of static pressure during the sintering process helps to eliminate pores and increase the density of ceramics, but it is difficult to completely bond special ceramics with ionic and covalent bonds. The elimination of pores in the material still has certain limitations for the desired ultra-high strength, high toughness, high hardness and high reliability materials. The main reasons for the limitations of HP static pressure sintering are reflected in the following three aspects: Before the start of sintering and the early stage of sintering, the constant pressure cannot make the powder in the mold fully realize the particle rearrangement to obtain a high bulk density; in the middle and late sintering period, Plastic flow and the elimination of agglomerates are still subject to certain restrictions, and it is difficult to achieve complete uniform densification of the material; in the later stage of sintering, it is difficult to completely eliminate residual pores under constant pressure.
 
Research on the mechanism of OPS technology to strengthen the densification of ceramics shows that:
 
First, the continuous oscillating pressure applied during the sintering process shortens the diffusion distance through particle rearrangement and elimination of particle agglomeration; secondly, in the middle and late sintering process, the oscillating pressure provides a greater sintering driving force for powder sintering, which is conducive to accelerating the viscosity. Flow and diffusion creep, stimulate the rotation of grains in the sintered body, grain boundary slip and plastic deformation to accelerate the densification of the green body; in addition, by adjusting the frequency and size of the oscillating pressure to enhance plastic deformation, it can promote the grain boundary at the later stage of sintering The combination and discharge of pores completely eliminates the residual pores inside the material, making the density of the material close to the theoretical density; finally, OPS technology can effectively inhibit the growth of grains and strengthen the grain boundaries. In short, the densification of materials in the OPS process mainly stems from the following two mechanisms:
 
The first is the traditional mechanisms of grain boundary diffusion, lattice diffusion, and evaporation-condensation under the action of surface energy; the second is the new mechanism conferred by oscillating pressure, including particle rearrangement, grain boundary slip, plastic deformation, and grain movement caused by deformation , Stoma discharge, etc. Therefore, the use of OPS technology can fully accelerate powder densification, reduce sintering temperature, shorten holding time, inhibit grain growth, etc., thereby preparing cemented carbide materials and ceramic materials with ultra-high strength and high reliability to meet extremes The application environment has higher requirements for material performance.
 
Summary
 
As an important member of engineering materials, advanced ceramic materials are still widely used by traditional sintering technologies, such as high energy consumption, high time consumption, and poor product performance. The new rapid sintering technology has demonstrated its huge advantages in the rapid preparation of special ceramics, material connection, gradient and nano-ceramic preparation.

 

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Hits:  UpdateTime:2021-09-28 17:21:50  【Printing】  【Close

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