Zirconium dioxide (ZrO2), also known as zirconia, is a versatile inorganic non-metallic material that exhibits exceptional properties such as high melting point, excellent corrosion resistance, wear resistance, and outstanding electrical conductivity.
Beyond its traditional applications in refractory materials and ceramic pigments, ZrO2 finds extensive use in advanced fields like electronic ceramics, functional ceramics, and structural ceramics.
Physical and Chemical Properties of Zirconium Dioxide
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Physical Properties
Zirconium dioxide boasts a high melting point, high boiling point, high hardness, and excellent electrical insulation properties at room temperature. However, at elevated temperatures, it exhibits remarkable electrical conductivity.
ZrO2 exists in three crystalline forms, making it a polymorphic oxide. The stable low-temperature phase has a monoclinic crystal structure (m-ZrO2). Above 1000°C, the tetragonal phase (t-ZrO2) gradually forms and persists until 2370°C. Beyond 2370°C and up to its melting point, the cubic phase (c-ZrO2) dominates. During the heating process, ZrO2 undergoes volume shrinkage, while it expands during cooling. To prevent these volume changes during use, crystal phase stabilization is necessary. Common stabilizers include Y2O3, CaO, MgO, CeO2, and other rare earth oxides. These oxides have cation radii similar to Zr4+ (within 12% difference) and exhibit high solubility in ZrO2, forming monoclinic, tetragonal, and cubic solid solutions. Rapid cooling of these solid solutions prevents decomposition, maintaining a metastable state at room temperature. The rapidly cooled cubic solid solution remains stable without undergoing phase transformation or volume change. This type of ZrO2 is called fully stabilized zirconia (FSZ).
Based on the characteristic conditions of ZrO2 phase transformation and the effects of different stabilizers, the effective addition (molar fraction) of stabilizers such as Y2O3, CaO, MgO, and CeO2 is typically 7-14%, 15-29%, 16-26%, and >13%, respectively. Depending on the application requirements, stabilizers can be used individually or in combination to obtain ZrO2 products with diverse properties.
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Chemical Properties
Zirconium dioxide exhibits excellent chemical stability. As a weakly acidic oxide, it demonstrates sufficient stability against alkaline solutions and many acidic solutions (except hot concentrated H2SO4, HF, and H3PO4). Crucibles made from ZrO2 can be used to melt various metals like potassium, sodium, aluminum, and iron. It is also stable against sulfides, phosphides, and many silicate melts and slags.
However, at high temperatures, molten alkaline silicates and molten silicates containing alkaline earth metals can corrode sintered ZrO2. Strong alkalis react with ZrO2 at elevated temperatures to form corresponding zirconates. In a vacuum at high temperatures (above 2220°C), ZrO2 reacts with carbon to form ZrC, and with hydrogen or nitrogen to form corresponding hydrides or nitrides.
Preparation Methods of Zirconium Dioxide Powders
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Co-precipitation Method
The co-precipitation method is the most widely used technique for preparing nano-sized zirconium dioxide due to its simplicity, easy control of the reaction process, and low cost. The process involves adding a portion of the stabilizer (e.g., Y(NO3)3) and dispersant (e.g., PEG2000) to a soluble zirconium salt solution (e.g., ZrOCl2·8H2O, ZrCl4, or Zr(NO3)4). A precipitant (e.g., NH3·H2O, NaOH, H2NCONH2) is gradually added to the salt solution while controlling the pH value. The reaction precipitates zirconium hydroxide gel and yttrium hydroxide gel, which undergo aging, filtration, water washing, alcohol washing, drying, and calcination to obtain zirconium dioxide powder.
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Hydrothermal Method
In the hydrothermal method, a soluble zirconium salt (e.g., ZrOCl2·8H2O, ZrCl4) is mixed with ammonia solution, and the pH is controlled. The reaction yields zirconium hydroxide gel, which is filtered, washed, and dried to obtain a hydrothermal precursor. The precursor is mixed with distilled water under controlled hydrothermal conditions to obtain a hydrothermal product. After filtration, washing, and drying, ZrO2 powder is obtained.
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Microemulsion Method
The microemulsion method involves mixing an aqueous solution of ZrOCl2·8H2O and Y(NO3)3 with ammonia and a mixture of cetyltrimethylammonium bromide and ethanol to form a reverse micelle solution. The reverse micelle solutions are mixed, stirred, and precipitated. The precipitate is filtered, washed, dried, and calcined to obtain zirconium dioxide powder.
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Electrofusion Method
The electrofusion method is an effective way to prepare zirconium dioxide powder due to its simplicity, low contamination, and low cost. The main process involves mixing zirconium-containing ore (e.g., zircon sand), carbon-containing materials (e.g., graphite, coke), stabilizers (e.g., yttrium oxide, calcium oxide), and clarifying agents (e.g., iron, aluminum oxide) uniformly. The mixture is then melted in an electric arc furnace at high temperatures. The molten liquid is cooled and further processed by grinding to obtain zirconium dioxide powder.
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Other Methods
Other methods, such as the sol-gel method, are also used to prepare zirconium dioxide powders.
Applications of Zirconium Dioxide
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Zirconium Dioxide Refractories
(1) Zirconium Dioxide Crucibles
With a melting point of 2700°C, zirconium dioxide crucibles can successfully melt platinum group metals and their alloys, potassium, sodium, quartz glass, oxides, and salts without reacting with the molten metals, silicates, or acidic slags, even at temperatures above 1900°C.
(2) Zirconium Dioxide Refractory Fibers
Zirconium dioxide fibers are the only ceramic fibers that can withstand ultra-high temperatures above 1600°C for extended periods. They exhibit higher use temperatures and better thermal insulation properties compared to alumina, mullite, and aluminosilicate fibers. They also possess excellent high-temperature chemical stability, corrosion resistance, oxidation resistance, low volatility, and non-polluting characteristics.
(3) Zirconium Dioxide Kiln Materials
Zirconium dioxide is used as a refractory material in critical parts of large glass tank furnaces. Early zirconia refractories contained only 33-35% zirconium dioxide. Asahi Glass Co., Ltd. of Japan successfully developed zirconia refractories containing 94-95% zirconium dioxide, significantly extending the service life of glass furnaces when used in the top and critical parts.
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Zirconium Dioxide Structural Ceramics
(1) Zirconium Dioxide Ceramic Bearings
Zirconium dioxide all-ceramic bearings exhibit excellent properties such as magnetic and electrical insulation, wear resistance, corrosion resistance, self-lubrication without oil, high-temperature resistance, and low-temperature resistance. They are suitable for extremely harsh environments and special working conditions.
(2) Zirconium Dioxide Ceramic Valves
Zirconium dioxide ceramic valves excel in this field due to their excellent wear resistance, corrosion resistance, and high-temperature thermal shock resistance.
(3) Zirconium Dioxide Grinding Materials
Zirconium dioxide grinding balls have high hardness, low wear rate, long service life, and can significantly reduce contamination of the grinding materials, ensuring product quality. The high density of zirconium dioxide materials results in strong impact energy when used as grinding media, greatly improving grinding and dispersion efficiency and effectively reducing grinding time.
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Zirconium Dioxide Functional Ceramics
(1) Zirconium Dioxide Ceramic Balls for Ball-Point Pens
(2) Zirconium Dioxide Ceramic Knives
Zirconium dioxide ceramic knives possess high strength, wear resistance, oxidation resistance, corrosion resistance, and do not react with food. They have a jade-like luster and are considered ideal high-tech green cutlery. Current market products include zirconium dioxide ceramic dining knives, scissors, razors, surgical knives, etc., which have gained popularity in Europe, America, Japan, and South Korea in recent years.
(3) Zirconium Dioxide High-Temperature Heating Elements
Zirconium dioxide is an insulator at room temperature with a high resistivity of 1015 Ω/cm. It becomes conductive at 600°C and a good conductor above 1000°C. It can be used as a heating element at 1800°C, with a maximum working temperature of 2400°C. It has been successfully used in heating elements and equipment operating in oxidizing atmospheres above 2000°C. Research is also actively exploring its use as an electrode material for magnetohydrodynamic power generation.
(4) Zirconium Dioxide Bioceramics
Zirconium dioxide dental crowns have excellent transparency and luster due to the absence of a metal inner layer. They effectively avoid problems such as tooth sensitivity and gingival black lines. With sufficient masking ability, they can perfectly address the aesthetic needs of patients with severe tetracycline-stained teeth. The toughness of zirconium dioxide materials compensates for the brittleness of ordinary dental porcelain. They have good biocompatibility, do not irritate oral mucosa tissues, are easy to clean, and are currently considered high-quality dental crowns both domestically and internationally.
(5) Zirconium Dioxide Coating Materials
High-performance zirconium dioxide thermal barrier ceramic coating materials stabilized with Y2O3 and other stabilizers are mainly used in high-performance turbine aircraft engines.
(6) Zirconium Dioxide Communication Materials
Zirconium dioxide pin bodies are key components in PC-type fiber optic active connectors.
(7) Zirconium Dioxide Oxygen Sensors
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Zirconium Dioxide Decorative Materials
(1) Zirconium Dioxide Gemstones
Zirconium dioxide gemstones are divided into natural cubic zirconia and artificially synthesized cubic zirconia. Natural cubic zirconia is extremely rare in nature, making it a scarce gemstone material. Naturally formed cubic zirconia comes in a wide range of colors, and large, high-quality natural zircons are priced no less than equivalent diamonds, making them highly valuable and rare natural gemstones. Artificially synthesized cubic zirconia has good optical properties and serves as an affordable and beautiful diamond substitute.
(2) Zirconium Dioxide Ceramic Jewelry
Currently, there are several types of zirconium dioxide ceramic jewelry: (1) Silver jewelry with embedded zirconium dioxide, which includes a wide range of materials such as zirconium dioxide stones, industrial zirconium dioxide, high-purity zirconium dioxide, stabilized zirconium dioxide, ultrafine zirconium dioxide, zircon sand, and zircon powder. Silver-plated rhodium jewelry with embedded cubic zirconia is particularly popular among European customers. (2) Pure zirconium dioxide ornaments. (3) Ornaments with additional functions, such as ceramic watch cases, bezels, and straps.
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Other Applications of Zirconium Dioxide
(1) Formation of Composite Materials with Zirconium Dioxide
Zirconium dioxide can form composite materials with other materials, such as alumina and mullite, resulting in new materials with superior properties compared to single-phase materials.
(2) Additive in Ordinary Ceramics
In ceramic glazes, zirconium dioxide serves as a good auxiliary colorant for yellow-green pigments. To obtain high-quality vanadium-zirconium yellow pigments, high-purity zirconium dioxide must be used. In glaze manufacturing, pure zirconium dioxide can increase the high-temperature viscosity and expand the temperature range of high-temperature viscosity changes. When added at 2-3%, it improves the crack resistance of glazes. Due to its high chemical inertness, zirconium dioxide enhances the chemical stability and resistance to acid and alkali corrosion of glazes. It is sometimes used to produce opaque glazes.
(3) Raw Material for Preparing Zirconates
Zirconium dioxide is used as a raw material for preparing zirconates by reacting with metal oxides or metal carbonates. Zirconates have large molecular structures and various electrical properties, making them useful in high-temperature and electronic component applications.