What are the characteristics of advanced ceramic powder

The properties of zirconia ceramic powder are very important to the quality of the advanced ceramics. The “quality” here not only refers to the excellent performance and repeatability of the product, but also includes the excellent process performance and stability. In order to achieve this ideal state. Advanced ceramic powder should have the following characteristics.

(1) Precise chemical composition
Accurate chemical composition is a basic requirement, because for advanced ceramics. Different chemical compositions directly determine the phase and phase of the produced crystals. Bianeng, if the chemical composition deviates, the result will be unrecognizable. Such as PZT piezoelectric ceramics. When Zr:Ti=52:48, it is the phase boundary between the trigonal phase and the tetragonal phase. When the designed composition falls in the tetragonal phase region, the piezoelectric performance of the product corresponds to the tetragonal phase. If it deviates to the trigonal phase region Inside. Then the piezoelectric properties of the product will be very different from the design requirements, which do not meet the requirements of product quality; for example, in SrTiO, j, excessive SrO will cause liquid phase to appear during sintering. This leads to secondary recrystallization, while excess TiO2 hinders sintering.

(2) Good uniformity of chemical composition
Good chemical composition uniformity means that the chemical composition is uniformly distributed. If the chemical composition is not uniformly distributed, it will lead to the deviation of the local chemical composition, thereby affecting the sintering of the powder and the performance of the product. Due to the uneven distribution of components, the local melting point is often lowered, the liquid phase appears, which promotes the growth of grains, or causes secondary recrystallization, or local difficulty in sintering, resulting in uneven microstructure of the product. If the density is not high, it will have an adverse effect on the performance of the product. As a result, the performance of the finished ceramic products is reduced, and the repeatability and consistency are deteriorated.

(3) High purity

High purity requires that the impurity content in the powder should be low, especially the harmful impurity content should be as low as possible, because the existence of impurities will affect the performance of the powder and the physical properties of the product. In order to ensure the purity of the powder, firstly, when selecting raw materials, it should be strictly controlled; secondly, in the preparation process, the introduction of harmful impurities should be avoided as much as possible. The chemical composition of the powder is related to whether the various properties of advanced ceramics can be guaranteed. The impurity content in the material also affects the sintering process to varying degrees. Although impurities are not necessarily harmful, there is usually a purity requirement for powders. For powders with insufficient purity, it should be hanged or used with caution. However, it should be pointed out that there should also be reasonable requirements for the purity of raw materials. We should not blindly pursue unnecessary purity and cause economic waste. We should try to use low-cost raw materials on the premise of satisfying product performance. Specific analysis of impurities is required, and some are not only harmless, but beneficial. For example, some impurities can form a eutectic with the main component to promote sintering; there are III, V group or II, VI group impurities that can compensate for ion valence and improve electrical properties. This is the main reason why products with the same performance are often not obtained by using different batches of raw materials of the same purity. When changing the batch number or origin of silicon carbide ceramic raw materials, in addition to its purity, attention should also be paid to the type and content of impurities, and analyze the possible effects on the product. And confirmed by small batch experiments.

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(4) Appropriately small particle size
The particle size of powder is an important factor determining its sintering performance. At a certain sintering temperature, the sintering rate is inversely proportional to a certain power of the particle size (such as particle radius), so the particle size often determines the sintering of the powder. performance. The finer the particle size and the less complete the structure, the greater its activity (instability and sinterability), which can not only reduce the sintering temperature, but also broaden the sintering temperature range, which is more conducive to the sintering.

(5) The particles are spherical and uniform in size
The ideal shape of powder particles should be spherical, because spherical particles have good fluidity and high particle bulk density (the theoretical calculation value is 74%). The distribution of pores is uniform, so that in the process of forming and sintering and densification, the growth of crystal grains and the exclusion and distribution of gas can be effectively controlled to obtain products with uniform microstructure, excellent performance and good consistency. In addition, the particle size of the powder should be uniform and single, because the powder with large particle size difference is often not dense in the area with larger particles after sintering, and the large particles often become the nucleus of secondary recrystallization, resulting in abnormally long individual grains. large, which seriously affects the microstructure and product performance. Therefore, the smaller the difference in particle size, the better, the ideal is that all particles have the same size, that is, the so-called single-sized particles of ceramic rings. Uniform, high density, and single size distribution, so that it can effectively control the development of microstructure during the densification process. In fact, it is difficult to achieve the requirement of uniform and single size of powder particles, and it is only possible to make the particle size distribution very narrow on the particle distribution curve, that is to say, it can only achieve approximately uniform and single size.

(6) Good dispersibility and no agglomeration
The ideal powder should be composed of single primary particles without agglomerates. The so-called “primary particle” refers to the most basic particle in the powder. The agglomerate refers to a certain number of primary particles in the powder. Due to the interaction of seed preparation forces (such as electrostatic force, van der Waals force, capillary force formed by the existence of liquid between particles, etc.), they form with each other. Bonds with a certain strength; or due to the formation of a bottleneck between the particles through diffusion during the calcination process, the particles are connected by a solid-phase bridge to form secondary particles of a certain size. Usually due to the agglomeration of particles caused by electrostatic force, van der Waals force, etc., the bond between particles is weak, and the strength of the agglomerate is also low. During the molding process, a certain pressure can break the agglomerate; However, due to the mutual diffusion of particles, the agglomerates of solid bridges formed between particles have relatively high strength and are not easily broken. The former becomes soft agglomerates and the latter is hard agglomerates. These two small and homogeneous agglomerates have different internal particle bonding conditions. The particles in the soft agglomerates have weak contact and are easily destroyed and dispersed into primary particles, while the hard agglomerates are Solid bridges are formed between particles in agglomerates, which are difficult to be destroyed; therefore, powders with no agglomerates or less closed polymers are regarded as powders with good dispersion.

The above request for powder is ideal. Youyu requirements are mutually restrictive, and in fact it is difficult to fully achieve them. If the powder is too fine, it is easy to absorb impurities and easy to agglomerate. in practice. People can only continuously innovate and improve the powder preparation technology and methods, in order to try to approach this idealized requirement and find its balance point in the complex mutual constraints. It should be emphasized here that for the ceramic materials of Xiaotong’s ceramic parts, each has its own particularity by T. Then the performance requirements of its powder should also be emphasized. For example, for some functional ceramics such as BaTi3, PLZT, etc., because the performance requirements are more focused on their ferroelectricity and piezoelectricity, the chemical and structural characteristics of the powder should be emphasized. The requirements for a curved structure ceramics, such as tetragonal YTZP, Si3N4, etc., are the most important, so the requirements for powders are more focused on their physical properties and structural properties.