By Pei Ze Technology | 28 May 2019 | 0 Comments

Properties and applications of zirconia ceramic powder

Introduction
Zirconium oxide ceramics, ZrO2 ceramics, and Zirconia Ceramic have excellent melting point and boiling point, high hardness, insulator at normal temperature, and electrical conductivity at high temperatures.

Type characteristics
Pure ZrO2 is white, yellow or gray when containing impurities, generally containing HfO2, not easy to separate. The world's proven zirconium resources are about 19 million tons, and zirconia is usually made from zirconium ore. There are three kinds of crystalline forms of pure ZrO2 under normal pressure: monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2) and cubic (cubic) zirconia (c-ZrO2). The three crystal forms exist in different temperature ranges and can be converted into each other:
Temperature density
Monoclinic zirconia (m-ZrO2) <950 ° C 5.65 g / cc
Tetragonal zirconia (t-ZrO2) 1200-2370°C 6.10g/cc
Cubic zirconia (c-ZrO2) > 2370 ° C 6.27 g / cc
The above three crystalline states have different physical and chemical properties. In practical applications, in order to obtain the desired crystal form and performance, different types of stabilizers are usually added to make different types of zirconia ceramics, such as partially stabilized zirconia (partially stabilized zirconia, PSZ), when the stabilizer is CaO, MgO, or Y2O3, it is represented by Ca-PSZ, Mg-PSZ, Y-PSZ, and the like, respectively. Tetragonal zirconia consisting of metastable t-ZrO2 is called tetragonal zirconia polycrysta (TZP). When the stabilizers added are Y2O3 or CeO2, they are represented by Y-TZP, Ce-TZP, and the like, respectively.
Powder preparation
The production of zirconia ceramics requires the preparation of powders with high purity, good dispersion, ultrafine particles and narrow particle size distribution. There are many preparation methods for zirconia ultrafine powders. The purification of zirconia mainly includes chlorination and thermal decomposition, and alkali metals. Oxidative decomposition method, lime melting method, plasma arc method, precipitation method, colloid method, hydrolysis method, spray pyrolysis method, and the like. The powder processing methods include a coprecipitation method, a sol-gel method, an evaporation method, a supercritical synthesis method, a microemulsion method, a hydrothermal synthesis method network, and a vapor deposition method.
Production Process
forming
The zirconia ceramics are formed by dry pressing, isostatic pressing, grouting, hot die casting, tape casting, injection molding, plastic extrusion molding, colloidal solidification molding, and the like. The most widely used are injection molding and dry pressing.

(1) Grouting molding

The molding process of the grouting process includes a physical dehydration process and a chemical coagulation process, and the physical dehydration removes the moisture in the slurry by the capillary action of the porous gypsum mold. The chemical coagulation process is because the Ca2+ generated by the dissolution of the CaSO4 on the surface of the gypsum mold increases the slurry. The ionic strength in the material causes flocculation of the slurry. Under the action of physical dehydration and chemical coagulation, ceramic powder particles are deposited on the wall of the plaster mold. Grouting is suitable for the preparation of large-scale ceramic parts with complex shapes, but the quality of the blanks, including shape, density, strength, etc., is poor, workers are labor intensive and not suitable for automation.

(2) Hot press molding

Hot injection molding is to mix ceramic powder with binder (paraffin) at a higher temperature (60~100 °C) to obtain a slurry for hot die casting. The slurry is injected into the metal mold under the action of compressed air. Press cooling, demoulding to obtain a wax blank, the wax blank is dewaxed under the protection of an inert powder to obtain a green body, and the green body is sintered at a high temperature to form a porcelain. The hot injection molded green body has accurate size, uniform internal structure, small mold wear and high production efficiency, and is suitable for various raw materials. The temperature of the wax paste and the mold must be strictly controlled, otherwise it will cause under-injection or deformation, so it is not suitable for manufacturing large parts, and the two-step sintering process is complicated and energy consumption is high.

(3) Tape casting

Tape casting is to thoroughly mix the ceramic powder with a large amount of organic binder, plasticizer, dispersant, etc. to obtain a viscous slurry that can flow, add the slurry to the hopper of the casting machine, and control the thickness with a doctor blade. The feed nozzle flows out through the feeding nozzle, and after drying, a film blank is obtained. The process is suitable for preparing a film material, and a large amount of organic matter is added in order to obtain better flexibility, and the process parameters are strictly controlled, otherwise defects such as peeling, streaking, low film strength or peeling off are easily caused. The organic substances used are toxic and cause environmental pollution. Non-toxic or less toxic systems should be used as much as possible to reduce environmental pollution.

Degreasing and debinding
In addition to the dry pressing-based forming technology, other products formed by the process must be degreased and degreased before being sintered, because other processes besides dry pressing will add a certain proportion to the zirconium powder during molding. Plasticizers, these plasticizers must be removed after the product has been formed, otherwise it will have a serious quality impact on the sintered product. The plasticizer is mainly composed of paraffin wax and other polymer materials. These materials are required to exhibit good plasticity and fluidity at a certain temperature, and have certain toughness and strength at normal temperature.

The sintering methods that can be used for zirconia ceramics are generally: pressureless sintering, hot pressing sintering and reactive hot pressing sintering, hot isostatic pressing (HIP), microwave sintering, ultrahigh pressure sintering, discharge plasma sintering (SPS), in situ. Press molding and the like. Often based on pressureless sintering.

application
In terms of structural ceramics, zirconia ceramics are widely used in the field of structural ceramics because of their high toughness, high flexural strength and high wear resistance, excellent thermal insulation properties, and thermal expansion coefficient close to that of steel. Mainly: Y-TZP grinding ball, dispersion and grinding media, nozzle, ball valve ball seat, zirconia mold, micro fan shaft, fiber pin, fiber sleeve, wire drawing die and cutting tool, wear-resistant tool, clothing button, Cases and straps, bracelets and pendants, ball bearings, golf ball hitters and other room temperature wear-resistant components.
In terms of functional ceramics, its excellent high temperature resistance is used as an induction heating tube, refractory material, and heating element. Zirconia ceramics have sensitive electrical performance parameters and are mainly used in oxygen sensors, solid oxide fuel cells (SOFC) and high temperature heating bodies. ZrO2 has a high refractive index (N-21^22), and it can be made into a colorful translucent polycrystalline ZrO2 material by adding certain coloring elements (V2O5, MoO3, Fe2O3, etc.) to the ultrafine zirconia powder. , like natural gemstones, sparkling with colorful light, can be made into a variety of decorations. In addition, zirconia is widely used in thermal barrier coatings, catalyst carriers, medical, health care, refractory materials, textiles, and the like.

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