Zirconium (IV) oxide
|__ Zr 4+ __ O 2−|
|Surname||Zirconium (IV) oxide|
|Ratio formula||ZrO 2|
colorless, odorless solid
|External identifiers / databases|
|Molar mass||123.22 g mol −1|
monoclinic: 5.7 g cm −3
2680 ° C
approx. 4300 ° C
1 mg / l (20 ° C)
1 mg m −3
|As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .|
Zirconium (IV) oxide (ZrO 2 ), zirconium dioxide , also known colloquially as zirconium oxide (older names are zirconic acid or zirconium earth), is the most common compound of the element zirconium in nature after zirconium .
Technical zirconium dioxide ZrO 2 is an inorganic material from the group of oxides . It is used as a powder u. a. Used for the production of high-performance ceramics ( oxide ceramics ) and as a single-crystalline artificial gemstone as jewelry and in optics.
Extraction and presentation
Zirconium silicate ZrSiO 4 ( zirconium ) is used as the starting product for the production of zirconium dioxide . This silicate sand is separated from impurities by washing, cleaning and calcining processes and converted into zirconium dioxide. A 99 percent pure zirconium dioxide powder is obtained in this way.
It is also formed during the dehydration and subsequent annealing of zirconium oxide hydrates or salts of zirconium such as nitrates, oxalates, acetates etc. with volatile, oxygen-containing acids.
Zirconium dioxide is diamagnetic , very resistant to acids and alkaline solutions and has a high resistance to chemical, thermal and mechanical influences. The chemical behavior is strongly dependent on the thermal pretreatment. When heated slightly, it dissolves in mineral acids fairly easily. After intense heating, it is soluble in concentrated sulfuric acid in addition to hydrofluoric acid and after melting it is only attacked by hydrofluoric acid. It is easily digested in melts of alkali hydroxide or carbonate, with which it forms zirconates which are soluble in acid.
Zirconia comes in three modifications :
- at room temperature it crystallizes in the monoclinic space group P 2 1 / c (space group no. 14) with a concentration number ( coordination number ) of zirconium with respect to oxygen of 7 (baddeleyite) and the lattice constants a = 5.138 Å , b = 5.204 Å, c = 5.313 Å, β = 99.2 °.
- Above 1170 ° C, it crystallizes in the tetragonal space group P 4 2 / nmc (No. 137) with a KZ of 8 (tetragonally distorted fluorite type)
- Above 2370 ° C it crystallizes in the cubic space group Fm 3 m (No. 225) with a KZ of 8 ( fluorite type)
The coefficient of thermal expansion depends on the modification of the zirconium dioxide:
- monoclinic: 7 · 10 −6 / K
- tetragonal: 12 · 10 −6 / K
- Y 2 O 3 -stabilized: 10.5 · 10 −6 / K
The addition of other metal oxides stabilizes the high-temperature modification at low temperatures. Properties such as B. The strength and translucency of these high-temperature modifications can thus be stabilized at room temperature. A proportion of at least 16 mol% calcium oxide (CaO) or 16 mol% magnesium oxide (MgO) is sufficient for crystallization in the cubic phase at room temperature. For a long time it was assumed that 8–8.5 mol% yttrium oxide (Y 2 O 3 ) (“8YSZ”) would be sufficient to stabilize the cubic phase at temperatures of up to 1000 ° C. It has been found in recent years that this is not the case (see paragraph on ionic conductivity). At least 9–9.5 mol% at 1000 ° C are necessary. At lower Y concentrations, metastable phases and mixed crystals form from the cubic and monoclinic phase. They generate internal prestress in the structure and ensure good thermal resistance to changes.
Common names and product names:
- partially stabilized ZrO 2 :
- PSZ, partly stabilized zirconia
- TZP, English: tetragonal zirconia polycrystal
- 4YSZ: ZrO 2 partially stabilized with 4 mol% Y 2 O 3 , English: yttria stabilized zirconia
- fully stabilized ZrO 2 :
- FSZ, English: fully stabilized zirconia
- CSZ, cubic stabilized zirconia
- 8YSZ: ZrO 2 fully stabilized with 8 mol% Y 2 O 3
- 8YDZ: 8–9 mol% Y 2 O 3 -doped ZrO 2 (same material as 8YSZ, naming due to the fact that 8YSZ is not completely cubically stabilized and chemically and microstructurally decomposes at temperatures up to 1200 ° C)
Translucent mixed crystals are called zirconia (also diamond imitation ) in the jewelry industry .
Oxygen ion conductivity and its degradation
Zirconium ions generally have a valence of +4 in the YSZ. Doping with oxides of metals of lower valency creates oxygen defects through the maintenance of the charge neutrality in the crystal, for example when adding Y 2 O 3 . The Y 3+ ions replace Zr 4+ on the cation lattice as follows:
This means that one oxygen vacancy is created for every two Y 3+ ions. The hopping of oxygen ions onto these vacancies in the electrical field enables high oxygen conductivity with a high electrical resistance for electron transport at the same time (8YSZ> 1 S / m, ref. And publications cited there). Due to these optimal transport properties, YSZ is used as a solid electrolyte e.g. B. used in high temperature fuel cells. It has been observed that, although not completely cubic stabilized, 8YSZ / 8YDZ has the highest conductance value for oxygen ions in the Y 2 O 3 -ZrO 2 system almost independently of the temperature in the range between 800 and 1200 ° C (Ref. And mentioned therein Publications). Unfortunately, it has been found in recent years that 8-9 mol% YSZ is operated at temperatures up to above 1200 ° C. in a miscibility gap of the Y 2 O 3 -ZrO 2 system and is therefore on the nm scale segregated into Y-depleted and enriched areas within a few 1000 hours. This chemical and microstructural segregation is directly linked to the drastic decrease in oxygen conductivity (degradation of 8YDZ) of around 40% at 950 ° C within 2500 hours.
It has also been found that traces of impurities or undesirable transition metals, e.g. B. Ni (from fuel cell production) can drastically increase the rate of segregation, so that segregation and degradation can play a decisive role even at lower operating temperatures of around 500–700 ° C. Therefore, multiply doped zirconium oxides are increasingly being used as electrolytes (e.g. scandium-yttrium codoping).
Zirconium oxide powder is added to paints to improve the properties (especially scratch resistance), e.g. B. automotive varnishes (topcoats), parquet varnishes, furniture varnishes, varnishes for electronic devices, nail varnishes. Inks for inkjet printers also contain zirconia.
Ceramic is produced from zirconium oxide powder by sintering and / or hot isostatic pressing . (Partially) stabilized zirconium dioxide is used as a refractory ceramic , as a technical ceramic in mechanical engineering and as a prosthetic material in medical technology due to its good thermal resistance and good mechanical properties .
Due to its ability to conduct oxygen ions electrolytically at higher temperatures (from approx. 600 ° C, oxygen ions can easily diffuse through vacancies in the crystal lattice ), zirconium dioxide is used as a solid electrolyte e.g. B. used in fuel cells (see section on oxygen ion conductivity ). An early application therefore found zirconia ceramic as a material for the filament (Nernst glower) of the Nernst lamp , one of Walther Nernst invented in 1897 incandescent type transmission. The oxygen ion conduction is also used to different oxygen partial pressures z. B. to measure between exhaust gases and air to determine the combustion coefficient ( lambda probe ). The property is also used in sensors or analyzers for measuring the oxygen content of gases. Yttrium-stabilized zirconium (IV) oxide (YSZ) is used for fuel cells and lambda sensors.
Zirconium (IV) oxide ceramic is used in medicine and the like. a. Used for the production of hip joint implants and in dentistry as a material for the production of crown and bridge frameworks, tooth-colored monolithic crowns and bridges, root posts and dental implants that do not contain any elemental metals or metal alloys. Zirconia ceramic is also used in orthodontic treatments to make brackets for fixed appliances. A primary telescope can be produced from zirconium dioxide ceramic for telescope prostheses.
Yttrium-stabilized zirconium (IV) oxide (YSZ) is also used as a ceramic material in medicine and in turbine technology.
Zirconium dioxide ceramic is used to manufacture rolling elements for hybrid bearings and all-ceramic bearings.
Due to its good abrasion resistance and chemical resistance, zirconium oxide is used in mills, e.g. B. as grinding balls
Man-made zirconium oxide single crystals are used as gemstones and as a material for optical components. Its high refractive index (2.15 at 643 nm wavelength) and its transparency in the 0.37–7 µm wavelength range are decisive for this.
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