What are the characteristics of advanced ceramic powders

The properties of zirconia ceramic powder are very important for the quality of preparing advanced ceramics; Quality; In addition to referring to excellent product performance and repeatability, it also includes excellent process performance and stability. To achieve this ideal state, advanced ceramic powders should have the following characteristics.

(1) Accurate chemical composition

Accurate chemical composition is a fundamental requirement, as for advanced ceramics, different chemical compositions directly determine the phase of crystal production. Your Majesty, if there is a deviation in chemical composition, the result will be completely unrecognizable. For example, PZT piezoelectric ceramics. When Zr: Ti=52:48, it is the phase boundary between the tripartite phase and the tetragonal phase. When the designed composition falls within the tetragonal phase zone, the piezoelectric performance of the product corresponds to the tetragonal phase. If it deviates to the tripartite phase zone, the piezoelectric performance of the product will be significantly different from the design requirements, which does not meet the requirements of product quality; In SrTiO and j, excessive SrO can cause liquid phase during sintering, leading to secondary recrystallization, while excessive TiO2 can hinder sintering.

(2) Good chemical composition uniformity

Good uniformity of chemical composition means that the distribution of chemical composition is uniform. If the distribution of chemical composition is not uniform, it will lead to deviation of local chemical composition, thereby affecting the sintering of powder and the performance of products. Due to uneven distribution of components, local melting points are often reduced, liquid phases appear, promote grain growth, or lead to secondary recrystallization, or local difficulty in sintering, resulting in uneven microstructure of the product. Low density will have a negative impact on the performance of the product, resulting in a decrease in the performance of ceramic products and a decrease in repeatability and consistency.

3) High purity

High purity requires a low impurity content in the powder, especially harmful impurities, as the presence of impurities will affect the technical performance of the powder and the physical properties of the product. To ensure the purity of the powder, strict control should be exercised when selecting raw materials; Secondly, during 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 presence of impurities in the material also has varying degrees of impact on the sintering process. Although impurities may not necessarily be harmful, there is usually a purity requirement for powder. For powder with insufficient purity, it should be avoided or used with caution. However, it should be pointed out that there should also be reasonable requirements for the purity of raw materials, and unnecessary purity should not be blindly pursued, resulting in economic waste. On the premise of meeting product performance, low-cost raw materials should be used as much as possible. For impurities, specific analysis should be conducted, and some are not only harmless, but also beneficial. For example, some impurities can form low eutectic compounds with the main components to promote sintering; Impurities of group III, V, or group II, VI can be used as ion valence compensation to improve electrical performance, etc. This is the main reason why different batches of raw materials with the same purity are often unable to obtain products with the same performance. When changing the batch number or origin of silicon carbide ceramic raw materials, in addition to paying attention to their purity, attention should also be paid to the type and content of impurities, analyzing the possible impact on the product, and confirming it through small-scale experiments.

(4) Appropriate small particle size

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

(5) The particles are spherical and uniform in size

The most ideal shape of powder particles should be spherical, as the spherical particle powder has good flowability, high particle stacking density (theoretical calculation value is 74%), and uniform pore distribution. Therefore, during the forming and sintering densification process, the growth of grains and the elimination 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, as powder with large particle size differences often has areas with larger particles that are not dense after sintering, and large particles often become the nucleus of secondary recrystallization, leading to abnormal growth of individual grains, which seriously affects the microstructure and product performance. Therefore, the smaller the difference in particle size, the better. The most ideal is that all particles have the same size, that is, the so-called Filter (aquarium)#Materials suitable for aquarium filtration single size particles. If such powder is not agglomerated, its molding performance is good. The obtained green body has uniform microstructure, high density, and single gas} L size distribution, so that it can effectively control the development of microstructure during the densification process. In fact, the requirement of uniform and single particle size in powder is difficult to achieve, and can only be achieved on the particle distribution curve, making its particle size distribution very narrow, that is, it can only achieve approximately uniform and single particle size.

(6) Good dispersibility without agglomeration

The ideal powder should be composed of individual primary particles, and there should be no agglomerates present. The so-called&quo; Primary particles; It refers to the most basic particles in the powder. Aggregates, on the other hand, refer to a certain number of primary particles in the powder that form bonds with a certain strength due to the interaction of seed preparation forces (such as electrostatic force, van der Waals force, capillary force formed by the presence of liquid between particles, etc.); Alternatively, during the calcination process, under the action of surface tension, bottlenecks are formed between particles through diffusion, resulting in solid phase bridging between particles and the formation of secondary particles of a certain size. Usually, due to the aggregation of particles caused by electrostatic forces, van der Waals forces, etc., the bonds between particles are weak, and the strength of their aggregates is also low. During the forming process, a certain amount of pressure can break these aggregates; Due to the mutual diffusion of particles, solid bridge aggregates are formed between particles, which have high strength and are not easily broken during molding. Strong methods such as adding dispersants, ball milling, and strong ultrasonic treatment are needed to break them. The former becomes a soft aggregate, while the latter is a hard aggregate. These two types of aggregates with similar properties have different particle bonding conditions inside them. The particles in the soft aggregate have weaker contact and are easy to be destroyed, leading to dispersion into primary particles. However, solid bridging is formed between the particles in the hard aggregate, making it more difficult to be destroyed; Therefore, powders without aggregates or with fewer closed aggregates are considered as well dispersed powders.

The above requirements for powder are idealized. The requirements are mutually restrictive and difficult to fully meet. For example, if the powder is too fine· Easy to adsorb impurities, easy to agglomerate, etc. In practice, people can only continuously innovate and improve powder preparation techniques and methods, in order to strive to meet the requirements of idealization and find a balance point in complex mutual constraints. It should be emphasized here that for the ceramic materials of Xiaotong ceramic parts, T has its own unique characteristics. Therefore, the performance requirements for their powders should also be emphasized. For some functional ceramics such as BaTi3, PLZT, etc., due to the emphasis on their ferroelectric and piezoelectric properties, the requirements for the chemical and structural characteristics of the powder are relatively high; For a curved structure ceramic, such as tetragonal YTZP, Si3N4, etc., its mechanical properties are the most important, so the requirements for powder materials are biased towards their physical and structural properties.

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