APPLICATIONS AND PREPARATIONS OF ZIRCONIUM OXIDE AND STABILIZED ZIRCONIA POWDERS

Technical Support Team

Stanford Materials Corporation, Aliso Viejo, California, USA

Pure Zirconium oxide (Zirconia) has a high melting point (2,700° C) and a low thermal conductivity. Its polymorphism, however, restricts its widespread use in the ceramic industry. During a heating process, zirconium oxide will undergo a phase transformation process. The change in volume associated with this transformation makes the usage of pure zirconium oxide in many applications impossible. Addition of some oxides, such as CaO, MgO, and Y2O3, into the zirconium oxide structure to a certain degree, result in a solid solution, which is a cubic form and has no phase transformation during heating and cooling. This solid solution material is termed as stabilized zirconia, a valuable refractory. Stabilized zirconia is used as a grinding media and engineering ceramics due to its increased hardness and high thermal shock resistivity. Stabilized zirconia is also used in applications such as oxygen sensors and solid oxide fuel cells due to its high oxygen ion conductivity.

1. Unstabilized (Pure) Zirconia

Pure zirconium Oxide is an important constituent of ceramic colors and an important component of lead-zirconia-titanate electronic ceramics. Pure zirconium oxide can be used as an additive to enhance the properties of other oxide refractories. It is particularly advantageous when added to high-fired magnesia and alumina bodies. It promotes sinterability and with alumina, contributes to abrasive characteristics. Pure zirconium oxide is monoclinic at room temperature and changes to the denser tetragonal form at about 1,000 °C, which involves a large volume change and creates cracks within its structures. Due to the inversion, pure zirconium oxide has low thermal shock resistivity.

2. Partially Stabilized Zirconia (PSZ)

Partially stabilized Zirconia is a mixture of zirconia polymorphs because insufficient cubic phase-forming oxide (Stabilizer) has been added and a cubic plus metastable tetragonal ZrO₂ mixture is obtained. A smaller addition of a stabilizer to the pure zirconia will bring its structure into a tetragonal phase at a temperature higher than 1,000 °C, and a mixture of cubic phase and monoclinic(or tetragonal)-phase at a lower temperature. Therefore, the partially stabilized zirconia is also called as tetragonal zirconia polycrystal (TZP). (see Fig.1)   Usually such PSZ consists of larger than 8 mol% (2.77 wt%) of MgO, 8 mol% (3.81 wt%) of CaO, or 3-4 mol% (5.4-7.1 wt%) of Y₂O₃. PSZ is a transformation-toughened material. Microcrack and induced stress may be two explanations for the toughening in partially stabilized zirconia. The Microcrack explanation depends upon difference in the thermal expansion between the cubic phase particle and monoclinic(or tetragonal)-phase particles in the PSZ. Coefficient of thermal expansion (CTE) for the monoclinic form is 6.5^-6/°C up to 1200 °C, 10.5^-6/°C for cubic form is. This deference creates microcracks that dissipate the energy of propagating cracks. The induced stress explanation depends upon the tetragonal-to-monoclinic transformation, once the application temperature over pass the transformation temperature at about 1000 °C. The pure zirconia particles in PSZ can metastabily retain the high-temperature tetragonal phase. The cubic matrix provides a compressive force that maintains the tetragonal phase. Stress energies from propagating cracks cause the transition from the metastable tetragonal to the stable monoclinic zirconia. The energy used by this transformation is sufficient to slow or stop propagation of the cracks. Partially Stabilized Zirconia has been used where extremely high temperatures are required. The low thermal conductivity (about 8 Btu/ft2/in/°F at 1800 °F) ensures low heat losses, and the high melting point permits stabilized zirconia refractories to be used continuously or intermittently at temperatures of ³ 2,200 °C (4000° F) in neutral or oxidizing atmospheres. Above 1,650°C (3000 ° F), in contact with carbon, zirconia is converted in to zirconium carbide. Zirconia is not wetted by many metals and is therefore an excellent crucible material when slag is absent. It has been used very successfully for melting alloy steels and the noble metals. PSZ refractories are rapidly finding application as setter plates for ferrite and titillate manufacture, and as matrix elements and wing tunnel liners for the aerospace industry. PSZ is also used experimentally as heat engine components, such as cylinder liners, piston caps and valve seats.

3. Fully Stabilized Zirconia

Generally, addition of more than 16 mol% of CaO (7.9 wt%),16 mol% MgO (5.86 wt%), or 8 mol% of Y₂O₃ (13.75 wt%), into zirconia structure is needed to form a fully stabilized zirconia. Its structure becomes cubic solid solution. Its structure becomes cubic solid solution (see Fig.1), which has no phase transformation from room temperature up to 2,500 °C. As a good ceramic ion conducting materials, fully yttria stabilized Zirconia (YSZ) has been used in oxygen sensor and solid oxide full cell (SOFC) applications. The SOFC applications have recently been attracting more worldwide attention, due to their high energy transfer efficient and environment concerns.

4. Preparation of Pure Zirconia Powders

Zirconia is usually produced from the zircon, ZrSiO4. To produce zirconia from zircon, the first step is to convert zircon to zirconyl chloride. It can be done by:

¯ Melting

¯ + HCl

There are two methods are used to make zirconia from the zirconyl chloride: thermal decomposition and precipitation.

A. Once the zircornyl chloride (ZrOCl₂ 8H₂O) is heated to 200 °C, it starts dehydration and becomes dehydrated ZrOCl₂. On next step, ZrOCl₂ decomposes into chlorine gas and becomes zirconia at a much higher temperature. Zirconia lumps obtained from the calcination then undergo a size reduction process, such as ball milling, into the particle size range needed, usually up to -325 mesh. This method is associates with low production cost. However, it is not easy to produce zirconia powders with high purity and fine particle size by the method.

B. Precipitation method, on other hand, uses chemical reactions to obtain the zirconia hydroxides as an intermediate. Its processing can be described as following:

¯

¯ +NH₄OH

¯ Wash

¯ Filtration

¯ Freezing Dry (Liquid N₂)

¯ Calcination

By this method, the grain size, particle shape, agglomerate size, and specific surface area can be modified within certain degree by controlling the precipitation and calcination conditions. Furthermore, its purity is also easier to be controlled. For the applications of zirconia in the slip casting, tape casting, mold injection and so forth, particle size and specific surface are important characteristics. Well-controlled precipitated zirconia powder can be fairly uniform and fine. Particle size can be made less than 1 µm.

5. Preparation of Stabilized Zirconia Powders

In order to achieve the requirement of the presence of cubic and tetragonal phases in their microstructure, stabilizers (magnesia, calcia, or yttria) must to be introduced into pure zirconia powders prior to sintering. Stabilized zirconia can be formed during a process called in-situ stabilizing. Before the forming processes, such as molding, pressing or casting, fine particles of stabilizer and monoclinic zirconia are well mixed. Then the mixture is used for forming of green body. The phase conversion is accomplished by sintering the doped zirconia at 1700 °C. During the firing (sintering), the phase conversion takes place.

High quality stabilized zirconia powder is made by co-precipitation process. Stabilizers are introduced during chemical processing, before zirconium hydroxide's precipitation. (See following flow chart):

¯ + Stabilizer (Y₂O₃, for example) + HCl

¯ + NH₄OH

¯ Wash

¯ Filtration

¯ Freezing Dry (Liquid N₂)

¯ Calcination

A cubic (or tetragonal) phase zirconia is formed during calcination of chemically precipitated intermediates. These powders have chemically higher uniformity than in-situ stabilizing powder and can be used in applications such as refractories, engineering ceramics and thermal barrier coatings.

5. Preparation of Stabilized Zirconia Powders

In order to achieve the requirement of the presence of cubic and tetragonal phases in their microstructure, stabilizers (magnesia, calcia, or yttria) must to be introduced into pure zirconia powders prior to sintering. Stabilized zirconia can be formed during a process called in-situ stabilizing. Before the forming processes, such as molding, pressing or casting, fine particles of stabilizer and monoclinic zirconia are well mixed. Then the mixture is used for forming of green body. The phase conversion is accomplished by sintering the doped zirconia at 1700°

C. During the firing (sintering), the phase conversion takes place.

High quality stabilized zirconia powder is made by co-precipitation process. Stabilizers are introduced during chemical processing, before zirconium hydroxide's precipitation. (See following flow chart):

A cubic (or tetragonal) phase zirconia is formed during calcination of chemically precipitated intermediates. These powders have chemically higher uniformity than in-situ stabilizing powder and can be used in applications such as refractories, engineering ceramics and thermal barrier coatings.

6. Considerations on Preparation of Precipitated Zirconia and PSZ powders.

Small particle, large specific surface area, desired particle size distribution will greatly enhance the sintering kinetics, such as shorter sintering time, lower sintering temperature and denser specific gravity of sintered body. However, the physical properties of powders also depend upon individual application. Forming processes such as tape casting and extrusion sometimes need a smaller specific surface area to enhance dispersion of the powders when mixed with solvents. Special efforts during the preparation of powders are needed to control initial particle size distribution, agglomeration, and calcination

A. Particle size: Particle size is mostly determined by the agglomerate's size that is formed in the early stages of powder preparation and during the precipitation. There is no or little effect on the particle size during drying or calcination stage, even calcinated at a different temperature or for different time periods.

B. Crystallite Size: Crystallite size is determined during Calcination due to the crystal growth. The calcination temperature has more significant effect on final crystal size than calcination time.

C. Specific Surface Area: As crystallite size, the specific surface area is strongly influenced by the calcination parameters, especially by calcination temperatures.

7. Summary

  There are many aspects, such as composition, impurity, particle size and specific surface area, must be taken into account, when one chooses a right zirconia or stabilized zirconia for his specific applications. Understanding of preparation processes, physical properties and phase transformation of zirconia and stabilized zirconia will definitely be advantages for the users. This article briefly discusses these aspects and gives reader some basic understanding of zirconia and stabilized zirconia.

Fig 1: The binary system of CaO-ZrO₂

Reference Information on Forming and Sintering of Yttria Stabilized Zirconia Powder Grade: PSZ-5.2YB-NB

Forming: