Zirconia: Synthesis and Characterization

Abstract

Main resource of zirconia is the mineral zircon which occurs in beach sand and placer deposits. Alkali fusion and thermal plasma dissociation are the frequently adopted procedures to convert zircon to zirconia. Synthesis of different zirconia phases (monoclinic, cubic, and tetragonal) can be accomplished by the precise control of different operating parameters and stoichiometry of the reagents. Mesoporous and nano-zirconia which find wide application in catalysis and electronics are synthesized by different methods like solution combustion synthesis, sol–gel synthesis, hydrothermal synthesis, co-precipitation, and solid-phase sintering. Recently, biosynthesis of zirconia has taken a quantum leap due to environmental concerns. The synthesized zirconia is characterized by various chemical, physical, and instrumental methods to find out composition, crystal structure, size, and morphology.

1. Introduction

Zirconia (ZrO₂) is one of the important materials which finds wide usage depending on its purity and crystal structure in various fields such as ceramics, refractories, electronics, and others. Diversified applications of zirconia as a high-technology material for industrial applications are due to its superior mechanical, thermal, electrical, chemical, and optical properties [1]. It is an ideal material for the production of ceramics, electronic materials, and pigment due to the combination of properties like hardness, strength, high melting point, and biocompatibility. Zirconia ceramics have excellent biocompatibility with the human body, a property which helped it to replace alumina for prosthesis devices in hip joints, femoral ball beads, and dental implants. Optically clear cubic zirconia, known as synthetic diamond, is widely used in jewelery.

Zirconium oxide exhibits three well-defined crystal structures, i.e., monoclinic, tetragonal, and cubic (Figure 1) [2]. The monoclinic phase is stable up to 1170°C, and above this temperature it is transformed into tetragonal phase. The tetragonal phase is stable up to 2370°C and then transforms to the cubic phase, which is stable up to the melting temperature of 2680°C. On cooling to the transformation temperature, the structure reverts back to the original phase [3]. Out of these, tetragonal to monoclinic transformation is of great importance due to large volume change. This volume reduction is very advantageous for improving strength and toughness of ceramics.

Figure 1.

Crystal structures of ZrO2: (a) cubic, (b) tetragonal, and (c) monoclinic. Red and blue spheres correspond to oxygen and zirconium atoms, respectively.

Baddeleyite (ZrO₂) is the naturally occurring zirconia, and it occurs in carbonatite rocks [4]. However, the major source of zirconia is its silicate mineral zircon (ZrSiO₄), and it mainly occurs as a constituent of beach sand and placer deposits along with rutile, ilmenite, and monazite. Beach sands of Australia, India, Brazil, and USA are rich in zircon. In terms of zirconia, world reserves of zirconium are around 64 million tones. Australia (35%) is the major producer of zircon followed by South Africa (28%), USA, and Mozambique (7% each) [5]. In placer deposits, zircon occurs as a major constituent along with ilmenite, rutile, and quartz, while the minor constituents are sillimanite, garnet, and magnetite. Zircon is separated from beach sands and placer deposits by physical beneficiation method as given in Figure 2.

Figure 2.

Physical beneficiation of beach sand [6].

Zircon has the general composition of 67% zirconia and 32.8% silica. Zircon is zirconium hafnium silicate mineral with general formula (Zr, Hf)SiO₄. Zircon usually contains some hafnium, typically about 1%. Table 1 gives the chemical composition of Egyptian zircon.

Oxide	% Composition
ZrO2	61.9
SiO2	30.6
Fe2O3	3.3
HfO2	1.2
ThO2	0.56
Na2O	0.12
MgO	0.18
Al2O3	0.87
Cr2O3	0.043
U3O8	0.039

Table 1.

XRF analysis data of chemical composition of Egyptian zircon [8].

Zircon is chemically very stable. Zircon is considered a refractive material due to its low coefficient of thermal expansion and high melting point. Extraction of zirconium, zirconia, and other products from zircon requires rigorous chemical and thermal treatments to break the bonds between ZrO₂ and SiO₂ (Figure 3) [1]. A variety of techniques have been proposed for the extraction of zirconia from zircon.

Figure 3.

Crystal structure of zircon [7].

2. Decomposition of zircon

The two major routes used for the decomposition of zircon are chemical and thermal process.

2.1 Chemical process

The chemical decomposition is divided into different categories based on the reagents used as fluxes. They are fused with:

1. Sodium hydroxide (caustic soda) [6, 8, 9, 10, 11].

2. Sodium carbonate [12].

3. Calcium oxide and magnesium oxide [13].

4. Calcium carbonate (lime) [14].

5. Carbochlorination [15].

2.1.1 Fusion with sodium hydroxide

This is the widely adopted commercial method for the extraction of zirconia from zircon sand. In this method, zircon is fused at 600°C with caustic soda to produce sodium zirconate (Na₂ZrO₃) and sodium silicate (Na₂SiO₃) (eq. 1):

$${\text{ZrSiO}}_4+\text{4NaOH}\to {\mathrm{Na}}_2{\mathrm{ZrO}}_3\downarrow +{\mathrm{Na}}_2{\mathrm{SiO}}_3+{\text{2H}}_2\mathrm{O}$$E1

Water-soluble sodium silicate is removed by washing, and the residual sodium zirconate is treated with hydrochloric acid to produce zirconyl oxychloride (ZrOCl₂-ZOC) (eq. 2):

$${\mathrm{Na}}_2{\mathrm{ZrO}}_3+\text{4HCl}\to {\text{ZrOCl}}_2+\text{NaCl}+{\text{2H}}_2\mathrm{O}.$$E2

Zirconium oxychloride obtained is further acidified to precipitate it as zirconium oxychloride crystals:

$${\text{ZrOCl}}_{2\left(\mathrm{l}\right)}+{\text{8H}}_2\mathrm{O}+\mathrm{HCl}\to {\text{ZrOCl}}_2.{\text{8H}}_2\mathrm{O}\left(\mathrm{s}\right)+\mathrm{HCl}.$$E3

The residual solids are separated by filtration which contains mainly sodium zirconate, hydrous zirconia, and some silica. This residue is dissolved in 5 M HCl at 90°C to obtain a clear solution of zirconium oxychloride. The zirconium oxychloride obtained is neutralized with ammonia to precipitate zirconium hydroxide which in turn on calcination at 900°C gives pure zirconia, assaying >99.5%, with a monoclinic structure.

Flow sheet of alkali fusion of zircon is given in Figure 4.

Figure 4.

Flow sheet for alkali fusion of zircon [16].

2.1.2 Fusion with sodium carbonate

In this method, zircon is fused with sodium carbonate at 1100°C for several hours in an electric furnace to produce sodium zirconium silicate (Na₂ZrSiO₅):

$${\text{ZrSiO}}_4+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{Na}}_2{\text{ZrSiO}}_5+{\mathrm{CO}}_2.$$E4

Sodium zirconium silicate reacts with excess of sodium carbonate to form sodium zirconate and sodium silicate according to the eqn:

$${\mathrm{Na}}_2{\text{ZrSiO}}_5+{\mathrm{Na}}_2{\mathrm{CO}}_3\to {\mathrm{Na}}_2{\mathrm{ZrO}}_3+{\mathrm{Na}}_2{\mathrm{SiO}}_3+{\mathrm{CO}}_2.$$E5

Sodium zirconate remaining after the washing is treated with HCl to produce zirconium oxychloride crystal (ZOC) which in turn is converted into zirconia.

2.1.3 Fusion with calcium oxide and magnesium oxide

This method is mainly adopted for the preparation of cubic zirconia. Mixtures of zircon and CaO/MgO in the same molar ratio are fused at 1200°C. The complete disintegration of zircon produces zirconia (ZrO₂) and calcium magnesium silicate according to the equation:

$${\text{ZrSiO}}_4+\mathrm{CaO}+\mathrm{MgO}\to {\mathrm{ZrO}}_2+{\text{CaMgSiO}}_4\downarrow .$$E6

Leaching of the fused mass with hydrochloric acid removes the silica totally. The zirconium oxychloride obtained upon calcination gives cubic zirconia. The temperature and the composition of CaO and MgO play an important role in the concentration of cubic and monoclinic zirconia obtained. At lower percentage of CaO/MgO mixture, monoclinic zirconia content increases whereas at higher CaO/MgO percentage cubic zirconia increases. The fusion of zircon with CaO and MgO separately results in the formation of monoclinic zircon, while their combination in the same molar ratio as zircon produces cubic zircon. At a lower temperature of 1200°C monoclinic zirconia content increases, while at 1400–1500°C cubic zirconia content increases.

2.1.4 Fusion with calcium carbonate (lime)

This method is not an industrial method for the decomposition of zircon. Zircon on fusion with lime and subsequent cooling, produce a very fine powder of calcium silicate and coarse crystals of calcium zirconate enabling the physical separation of the two an easy process:

$${\text{2ZrSiO}}_4+{\text{5CaCO}}_3\to {\text{2CaZrO}}_3+{\left(\mathrm{CaO}\right)}_3{\left({\mathrm{SiO}}_2\right)}_2+{\mathrm{CO}}_2.$$E7

Calcium zirconate is converted into ZOC by treating with HCl.

2.1.5 Carbochlorination

This reaction takes place in a chlorinator. Zircon is heated with chlorine gas at 1100°C by induction heating of the graphite walls of chlorinator. Products, consisting of zirconium tetrachloride, silicon tetrachloride, and carbon monoxide, which are in gas phase are cooled down to 200°C:

$${\text{ZrSiO}}_4+\text{4Cl}+\text{4C}\to {\text{ZrCl}}_4+{\mathrm{SiC}}_4+\mathrm{CO}.$$E8

On cooling, zirconium tetrachloride solidifies first followed by silicon tetrachloride facilitating their separation. Zirconium tetrachloride is converted into zirconium oxychloride on treatment with water.

Zircon decomposition by alkali fusion has the capability of large-scale production and high efficiency, in comparison with chloride and lime sintering method. But in the former method the temperature profile and atmospheric control inside the furnace plays a crucial role. Silica carryover to zirconium fraction increases at high temperature. Once the sodium silicate is extracted from the sodium zirconate and dissolved in hydrochloric acid, two distinct routes can be followed to precipitate various zirconium chemicals. The most common route is to precipitate zirconium oxychloride crystals (ZOC), with subsequent purification from all contaminants (crystal route). Less known is the process (liquid route) that involves the direct precipitation of zirconium basic sulfate (ZBS). This route will yield a less pure product, with contaminants such as silica and titanium. An important factor in this route is the prevention of silica gel formation, which could hamper final product filtration.

2.2 Thermal decomposition of zircon

Even though chemical decomposition by alkali fusion at very high temperature is the time-tested method for the extraction of zirconia from zircon, it has certain shortcomings. High treatment cost, complexity of the process, and environmental issues associated with effluent treatment are some of them. These drawbacks compelled the search for cost-effective and environmentally friendly alternatives for zircon decomposition. The promising thermal plasma technology offers a one-step alternative for this conversion. Use of thermal plasma dissociation for the decomposition of zircon was carried out first by Wilks and co-workers in 1966 [17]. Zircon is dissociated to ZrO₂ and SiO₂ when heated above 1700°C [18, 19] as expressed by the following reaction:

$${\text{ZrSiO}}_4\to {\mathrm{ZrO}}_2+{\mathrm{SiO}}_2.$$E9

The above reaction is reversible and the oxides recombine to form the silicate on cooling. However, if zircon is heated to temperatures exceeding 1900°K and the reaction products are quenched rapidly, the reversible reaction is prevented. Recombination of products back to zircon is prevented by rapid cooling. The silica can be separated from the product by simple acid or alkali leaching to get pure zirconium oxide [20]. For thermal plasma decomposition, a direct current non-transferred argon arc plasma torch is used to generate plasma [21, 22]. The thermal dissociation is thermodynamically reversible, and both oxides recombine to form the silicate [19]. But if the cooling rate is fast enough (quenched) the recombination is avoided, and the product of the dissociation is known as plasma-dissociated zircon (PDZ). During the heating process, the zirconium silicate crystal structure re-arranges into amorphous silica and zirconium oxide phases. On fast cooling, amorphous glassy silica matrix entraps the fine zirconium oxide particles which can be easily disintegrated by acid attack. At high temperature, the stable zirconia phase is the tetragonal zirconia (t-ZrO₂) as inferred from phase diagram of zirconia (Figure 5). The resulting tetragonal zirconia during the cooling might transform into monoclinic zirconia which is the stable phase at room temperature [23]. Plasma decomposition of zircon is greatly affected by forward power, plasma and carrier gas flow, and location of power feed port.

Figure 5.

Phase diagram of zircon [23].

The microstructure of PDZ is influenced by the cooling rate after melting which depends strongly on the initial zircon particle size [24]. Thus, relatively slow cooling results in spherulitic crystals of monoclinic ZrO₂ in SiO₂ glass, whereas the rapid cooling gives extremely fine (< 10 nm) crystals of tetragonal ZrO₂ crystals in glassy silica matrix. Major advantages of thermal plasma processing over conventional methods are the ability of plasma reactors to attain high energy density and high temperatures, ability to control the processing atmosphere, increased reaction kinetics, eco-friendly nature of the process, the rapid cooling rate which prevents back reaction, and adaptability to process a variety of materials [25].

Plasma thermal dissociation processes have undergone a variety of process changes to obtain zirconia with high purity and definite crystal structure. Low-power transferred arc plasma (TAP) in which air is used as plasma gas instead of conventional argon gas renders the process cost-effective [24]. Carbothermal plasma dissociation in which coke is added to zircon during plasma dissociation is another adaptation. Energy consumption and overall operating cost of production are drastically reduced by these processes compared to conventional methods which are energy-intensive and expensive [21]. In flight, removal of silica is achieved in carbothermal plasma dissociation [26]. The thermally aided dissociation process becomes a thermo-chemically driven reaction at a much lower temperature. Another major feature is that the oxides of Al, Fe, and Ti and Mg that are present as impurities in zircon mineral are completely removed as vapor during plasma processing.

3. Alternate methods for the synthesis of zirconia

The above-mentioned procedures are used generally for the synthesis of macro- or micro-crystalline zirconia which is mainly used as ceramics due to its hardness, abrasiveness, high melting point, and low frictional resistance. At nanoscale, it becomes immensely valuable owing to its high thermal stability, luminescence, refractive index, chemical stability, high specific area, biocompatibility, and ability to exhibit significant antibacterial, antioxidant, and antifungal properties. Such outstanding characteristics have motivated the scientific community to explore zirconia-based nanomaterials in a wide range of technological fields as functional materials, viz., catalysts [27], sensors [28], semiconductor devices [29], ceramics [30], and implants [31]. Apart from that, it can also be employed as a dielectric, electro-optic, and piezoelectric material due to its favorable optical and electrical properties. Zirconia nanoparticles are synthesized by chemical and physical methods.

3.1 Chemical methods for the synthesis of nano-zirconia

Important chemical methods used for the synthesis are solution combustion synthesis (SCS), sol–gel synthesis, hydrothermal synthesis, and co-precipitation.

3.1.1 Solution combustion synthesis (CSC)

Solution combustion synthesis (SCS) has been widely used for the synthesis of metal oxides with desired morphology. This involves the complex exothermic reactions in solution between fuel which is an organic compound and zirconium metal solution. SCS is a self-sustained redox reaction initiated by a source of energy (thermal or electric) between a fuel and an oxidant (usually metal nitrates) [32]. The fuel is typically composed by organics containing carbon and hydrogen that facilitate the liberation of heat by the formation of CO₂ and H₂O during the combustion process. The metal precursors are the source of the metal cations which decides the final metal oxide. One of the methods for the synthesis of zirconia nanopowders is by annealing stochiometric mixtures of zirconium oxy nitrate hydrate in alcohol and urea [33]. Nanocrystalline zirconium oxide powder has been prepared by the glowing combustion method using sucrose as the fuel and zirconyl nitrate as an oxidant in aqueous solution [34]. Solution combustion synthesis is an attractive technique for the preparation of ZrO₂ nanopowders and thin films, owing to its simplicity, energy and time savings, cost-effectiveness, versatility, and higher purity compared to conventional methods [35].

3.1.2 Sol–gel synthesis

Sol–gel synthesis is the stepwise formation of metal oxide nanoparticles in which the sol or colloidal solution of solid particles formed first is transformed into a gel which is an interconnected network of polymerized metal oxide in a solvent. Drying of the gel results in aerogels having nanocrystalline size [36]. For the preparation of zirconia by sol–gel process, zirconium alkoxide in alcohol (precusor) is hydrolyzed with ammonia. Resultant sol is stirred continuously until the gel is formed. The gel formed is dried in conventional oven or by supercritical drying methods. Supercritical drying produces aerogels with smaller size compared to conventional drying methods:

$$\mathrm{Zr}{\left({\mathrm{OC}}_3{\mathrm{H}}_7\right)}_4+{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH}+{\mathrm{NH}}_4\mathrm{OH}\to \mathrm{Zr}{\left(\mathrm{OH}\right)}_4\left(\mathrm{sol}\right)+{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH}+{\mathrm{NH}}_3.$$E10

$$\mathrm{Zr}{\left(\mathrm{OH}\right)}_{4\left(\mathrm{Sol}\right)}\left(\mathrm{on}\text{polycondensation}\right)\to {\left[\mathrm{Zr}{\left(\mathrm{OH}\right)}_4\right]}_{\mathrm{n}}.{\mathrm{xC}}_3{\mathrm{H}}_7\mathrm{OH}.{\mathrm{xH}}_2\mathrm{O}.$$E11

$${\left[\mathrm{Zr}{\left(\mathrm{OH}\right)}_4\right]}_{\mathrm{n}}.{\mathrm{xC}}_3{\mathrm{H}}_7\mathrm{OH}.{\mathrm{xH}}_2\mathrm{O}\left(110°\mathrm{C}\right)\to \mathrm{Zr}{\left(\mathrm{OH}\right)}_4+{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}.$$E12

$$\mathrm{Zr}{\left(\mathrm{OH}\right)}_4\left(400°\mathrm{C}\right)\to {\mathrm{ZrO}}_{2\left(\mathrm{T}\right)}\left(500-700°\mathrm{C}\right)\to {\mathrm{ZrO}}_{2\left(\mathrm{T}\right)}+{\mathrm{ZrO}}_{2\left(\mathrm{M}\right)}$$E13

In this method, the reactivity of the precursors can be modified using chelating agents, which influence the gelation and ultimately the modification of the size and shape of the particles. The different chelating reagents that have been employed for the preparation of nano-zirconia are acetylacetone, acetic acid, citric acid, and different sugars (e.g., sucrose, maltose, and glucose). The advantages of sol–gel synthesis over other techniques are the ease in controlling the purity, homogeneity, and physical characteristics at low temperature [37].

3.1.3 Hydrothermal synthesis

Hydrothermal method for the synthesis of nanocrystalline zirconia is usually carried out by heating a mixture of precursors (zirconium compounds) with appropriate reagent which are reducing/oxidizing/hydrolyzing in a suitable solvent in a closed vessel [31, 38]. This method ensures the production of nanocrystals with definite morphology, crystal structure, stability, and size. In some methods, surfactants are added to the mixture to avoid agglomeration. Agglomeration can be further eliminated by hydrothermal corrosion methods. This method involves the use of corroding mediums such as sulfuric and hydrochloric acid to break down the hard agglomerate to dispersed fine nanoparticles. By carefully adjusting the reaction parameters, zirconia with definite crystalline structure and required size can be obtained. By introducing suitable dopants at the synthesis stage, zirconia with different physical properties can be synthesized. By introducing Eu³⁺ during the synthesis of cubic zirconia, white light-emitting nano-zirconia is produced which can be used in light-emitting diodes and electronic flashes [39]. Zirconia nanocrystals with different size can be synthesized by careful control of reaction parameters and concentration of reactants. Spindle−/rod-like structures are synthesized by this method [40]. Surfactant-assisted hydrothermal synthesis of zirconia produces thermally stable zirconia crystals with different morphology [41]. Surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 are generally used for this type of synthesis. By tuning the reaction conditions during synthesis, definite phase composition and morphology can be achieved (Figure 6).

Figure 6.

Surfactant-assisted nano-zirconia synthesis [41].

3.1.4 Microwave-assisted hydrothermal synthesis

This is another method for the synthesis of nano-zirconia. The advantages of this method over the other methods are volumetric heating (the entire volume of solution is evenly heated, instead of relying on heat diffusion processes across the reaction vessels) and short reaction time (reaction time can be as short as a few minutes) [42]. Moreover, the accurate control of morphology and crystal structure is possible by adjusting microwave parameters. Different zirconia phases can be obtained by adjusting pH, temperature, reaction time, pressure, and the precursor used [43]. In this method, microwave digestion of stochiometric composition of zirconia percussor, generally a zirconyl compound (e.g., zirconyl chloride, zirconyl hydroxide, zirconyl nitrate hydrate, and zirconium alkoxides) and an alkali generally sodium hydroxide is carried out at specified temperature and pressure.

3.1.5 Co-precipitation technique

This is an easy method for the synthesis of zirconia nanoparticles. Co-precipitation method is a promising alternative to other methods due to its inherent simplicity, ecological compatibility, precise stoichiometry, structural control, and large-scale production. However, size, shape, and dispersion of powders prepared by this method depends strongly on precipitants used [44]. Hence, the selection of appropriate precipitating reagent is the most important factor in co-precipitation method. Ammonium hydroxide (NH₄OH), ammonium bicarbonate (NH₄HCO₃), ammonium carbonate [(NH₄)₂CO₃], sodium hydroxide (NaOH), and urea are the typical precipitants used in precipitation method. pH of the precipitating medium has significant influence on the homogeneity and composition of the precipitate [45]. Precipitants are added to zirconium precursor, typically zirconium oxychloride or nitrate at a controlled rate. As the critical solution concentration of the zirconium hydroxide is reached, nucleation starts followed by growth phase. Zirconium hydroxide on calcination gives zirconia nanoparticles. Morphology of the nanoparticle obtained can be controlled by the ratio of the reactants and calcination temperature [46]. Surface area of the zirconia prepared by this method depends on nature of the precursor, rate of addition of base, pH of the solution, and presence of surfactants and organic anions like oxalate and citrate [47]. At an increased calcination temperature, size of the crystal and its agglomeration increased.

3.2 Solid-phase sintering

In this method, precursors are mixed and milled in a high-intensity ball mill. Meso-porous zirconia with worm-like structure is obtained by milling zirconyl chloride mixed with block polymer surfactant along with sodium hydroxide. The mixture is autoclaved and allowed to crystallize [48]. Sintering method is used for the densification of the material. Preparation parameters have strong influence on the structure of zirconia synthesized. Pore structure changes from microporous to mesoporous with change in parameter. A controlled grain size with good densification can be achieved by proper selection of heating schedule. Densification of powders prepared by chemical vapor methods can undergo one step sintering at 1000°C. Powders prepared by solution technique cannot be sintered at a lower temperature. Sintering process depends on mass transport mechanism [49]. Four underlying mechanisms of sintering are surface diffusion, spread throughout volume boundary, evaporation, and condensation. Temperature and residence time of the material under sintering are decisive in particle size and bond strength. The smaller the particle size, greater will be the bond strength due to the increased contact between the particles (Figure 7).

Figure 7.

A model showing sintering process [50].

3.3 Biosynthesis of zirconia

Biosynthesis is a greener approach to zirconia synthesis which helps to enhance its biocompatibility and reduce the environmental concerns. This process involves the synthesis of zirconia nanoparticles by reducing, stabilizing, and capping of the metal precursor using natural and renewable agents like microbes and plant parts [51, 52, 53, 54, 55]. Major steps involved in this process are reduction and stabilization. There are intracellular and extracellular pathways adopted for reduction. In intracellular pathway, metal ion is transported into the cell by interaction with the negative receptors of the cell wall. The enzymes of the cell cause the reduction of metal. In extracellular synthesis, zirconium ions are reduced by an enzyme – nitrate reductase. After the reduction, the zirconia nanoparticles are formed via nucleation, aggregation, and growth (Figure 8). The protein species released by the cell wall act as capping agents which stabilize zirconia nanoparticles [56].

Figure 8.

Biosynthesis of zirconia [56].

4. Characterization of zirconia

Characterization is the essential requirement to understand the physical, chemical, structural, and morphological properties of a newly synthesized material. Characterization involves the identification of different components present in the material, their quantification, structural elucidation, and identification of morphological properties of the material. Quantification of different components in zirconia is carried out by chemical procedures and by instrumental techniques like X-ray photoelectron spectroscopy (XPS), wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Structural elucidation is carried out by instrumental techniques like X-ray diffraction (XRD) spectroscopy, Fourier-transformed infrared spectroscopy (FTIR), N2 adsorption measurements, and Raman spectrometry. Morphology of synthesized material is usually assessed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

4.1 Chemical characterization of zirconia

Complete chemical characterization of zirconia is the quantification of elements present in zirconia. Multielement analysis of zircon samples is usually carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma mass spectroscopy (ICP-MS) after the quantitative dissolution [57]. Zirconia being a highly refractory material, quantitative decomposition of the material cannot be accomplished by usual acid decomposition procedures. Fusion of zirconia with sodium carbonate, sodium metaborate, and lithium metaborate are reported methods for complete dissolution. Microwave-assisted digestion of zirconia with a mixture of hydrofluoric acid, sulfuric acid, and hydrochloric acid is also used for decomposition of zirconia. Once in solution state, the concentration of elements is determined by either ICP-AES or ICP-MS. Electrothermal vaporization inductively coupled plasma mass spectrometry (ET-ICP-AES) in presence of a modifier is an effective method for the characterization of high-purity zirconia [58]. ICP-MS has also been used for the single-particle analysis of zirconia colloids in water [57]. Laser ablation inductively coupled plasma mass spectrometry is an excellent technique for the chemical characterization of zirconia samples.

4.2 X-ray photoelectron spectroscopy

Surface chemical characterization of zirconia is carried out using XPS. XPS also known as electron spectroscopy for chemical analysis (ESCA) is a quantitative spectroscopic technique which gives valuable information on the surface elemental composition and chemical state of the components of the material under study [59]. XPS provides surface analysis data and the average depth of analysis for an XPS measurement is approximately 5 nm. XPS is typically accomplished by exciting a samples surface with mono-energetic Al kα or Mg Kα X-rays which prompts the emission of photoelectrons from sample surface. An electron energy analyzer is used to measure the energy of the emitted photoelectrons. From the binding energy and intensity of a photoelectron peak, the elemental identity and chemical state of the element can be determined. Surface defects can be inferred from chemical state of the elements on the surface. Wide-scan spectrum, core-level spectrum, and the upper valence band (UVB) spectrum are the major analysis modes used for characterization in XPS. The wide-scan spectrum gives elemental information of the surface. Figure 9a shows the wide-scan spectrum of zirconia. It shows peaks corresponding to Zr and oxygen. Presence of any peak other than that of Zr and O₂ shows contamination or doping. Figure 9b shows the core-level spectrum of zirconia. XPS core-level peaks are used to obtain information on the chemical state of the surface, and crystalline phase. Figure 10 gives the Zr-3d_5/2, Zr-3d_3/2, and O-1 s core-level peaks of thin films of zirconia with different oxygen content. The chemical shift observed in the films with higher oxygen content corresponds to different oxidation state of zirconia. Peak shift in core-level peaks, Zr-3d_5/2, Zr-3d_3/2, and O-1 s of Figure 11 corresponds to phase change.

Figure 9.

The wide-survey scan spectrum (a) and Zr 3d core-level XPS spectra (b) of zirconia [60].

Figure 10.

(a) Core-level Zr 3d (XPS) spectra of bulky metallic ([O/Zr] = 0.07), partially oxidized films ([O/Zr] = 0.25 and [O/Zr] = 0.9) thin oxide films([O/Zr] = 1.3) with different oxidation states: Zr⁰ for metallic, Zr¹⁺, Zr²⁺, Zr³⁺ for sub-oxides and Zr⁴⁺ for stoichiometric oxide. (b) Core-level O1s spectra (XPS) for the corresponding Zr3d spectra [61].

Figure 11.

XPS spectra of (a and b) tetragonal and (c and d) monoclinic zirconia films annealed at 650^oc and 850^oc [62].

Identification of different crystalline phases by shift in core-level peaks is extremely difficult as the variation in crystal structure does not produce large shift in the core levels. However, by analyzing the valence band of XPS, phase transformation data can be obtained. Figure 12 shows the upper valence band XPS (UVBXPS) spectrum of zirconia surfaces grown by dry thermal oxidation in the temperature range 300–450°K. Oxide films grown below 400°K are predominantly amorphous, and formation of tetragonal phase starts above this temperature. There is a pronounced change in the shape of upper valence band spectra with increase in oxidation temperature. This can be attributed to the gradual change in the formation of tetragonal phase (increase in crystallinity) with temperature above 400°K. Change in the shape of the spectra with crystallinity is due to the increase in Zr-O bond ionicity and changes in the first coordination spheres of both Zr and O.

Figure 12.

XPSUVB spectra of thermally grown ZrO₂ films at temperatures 300°K and 450°K [63].

4.3 Fourier-transformed infrared spectroscopy (FTIR)

FTIR spectrum is an effective tool for the identification of functional groups present in a molecule [64]. When infrared radiation passes through a molecule, it causes changes in the dipole moment of the molecule which corresponds to a definite vibrational energy. Since every functional group is composed of different atoms and bonds with different bond strengths, frequencies of vibrations are unique to individual groups and classes of functional groups (e.g., O-H and C-H stretching frequency appear around 3200 cm⁻¹ and 2900 cm⁻¹, respectively). Since the collection of vibrational energy bands for all the functional groups of a molecule is unique to every molecule, these peaks can be used for identification using library searches of comprehensive sample database. FTIR spectrum of ZrO₂ nanoparticles and surfactant-modified zirconia are shown in Figure 13. The absorption peak at 608 cm⁻¹ corresponds to Zr-O stretching vibrations. A shift in the position of this peak is observed in the surfactant-modified zirconia. Two extra broadbands at 1607 cm⁻¹ 1nd 1400 cm⁻¹ in the spectra of surfactant modified ZrO₂ are attributed to the bending vibration of C-H bonds.

Figure 13.

FTIR spectra of zirconia nanoparticles (a) and surfactant-modified zirconia nanoparticles (b) from 4000 to 400 cm⁻¹ [65].

4.4 BET surface area analysis

Brunauer–Emmett–Teller (BET) surface area analysis is used to measure total surface area, total pore volume, and pore size distribution of a material [64]. In this method, multipoint analysis of analytes-specific surface area (m²/g) is measured through gas absorption analysis. When inert gases like N2 is flown over the solid sample, the gas molecules absorb on to analyte surface and in pores due to the weak van der Waals forces forming a monolayer of gas on the surface. After the adsorption layers are formed, the sample is removed from the nitrogen atmosphere and heated to release the adsorbed nitrogen from the material and quantified. The data obtained is used to plot the BET isotherm which is a plot of amount of gas adsorbed as a function of pressure. At normal temperatures, the interaction between solid and gases is minimal, and the sample surface is cooled with liquid nitrogen to obtain appreciable adsorption. Surface area is calculated using BET equation from the adsorption branch of isotherm, and pore size distribution was calculated from desorption branches using the Barrett–Joyner–Halenda (BJH) method [64]. Figure 14 shows the BET adsorption–desorption curve of ZrO₂ nanoparticles. The surface area and size of the nanoparticle obtained from this curve are 44m²/g and 24 nm, respectively [66].

Figure 14.

BET adsorption desorption curve for zirconia nanoparticles [66].

4.5 X-ray diffraction spectroscopy (XRD)

Powder X-ray diffraction is a method used for the identification of crystalline structure and phases of a material [67]. When X-rays interact with crystalline samples, they are diffracted at a particular angle which satisfies Bragg’s equation (nλ =2dsinθ) which gives the relation between wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. The diffracted X-rays are detected, processed, and counted. By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice are obtained due to the random orientation of the powdered material. Conversion of these diffraction peaks to d spacing by comparing it with standard reference patterns allows the identification of crystalline structure. Quantitative characterization of lattice defects and relative stress endured by the crystals can also be elucidated by XRD. As zirconia exists in three different crystalline structures, ascertaining the crystalline phase of the newly synthesized material is the most important part of the characterization of zirconia. Crystalline structure of synthesized zirconia depends on so many factors like temperature of calcination, pH of synthesis, and the reagents used. Figure 15 shows ZrO₂ nanoparticles synthesized at different temperatures (400, 500, and 600°C). The well-defined peaks at 2θ = 30.2, 35.0, 50.4, 60.0, and 62.7°correspond to the diffractions of the (101), (110), (200), (211), and (202) crystalline planes of cubic zirconia. The shape of the peaks at 35.0, 50.4, and 60.0° are slightly asymmetric, suggesting the formation of a tetragonal phase. The shoulder at 2θ = 34.5° is due to the diffraction of the (002) crystalline plane of tetragonal phase zirconia. The appearance of peaks corresponding to tetragonal and cubic phase in the XRD spectrum on increasing the temperature from 400 to 600°C gives a clear indication of the conversion of amorphous to crystalline form with increase in temperature.

Figure 15.

XRD patterns of ZrO₂ NPs synthesized at different temperatures [68].

4.6 Raman spectroscopy

Raman spectroscopy is a powerful, nondestructive, user-friendly technique to determine the surface characteristics and subsurface damages. This technique is based on Raman effect which in-elastic scattering process which occurs when an incident photon interacts with phonons in the material [69]. When the photons of a particular wavelength interact with the vibrating phonons of the materials, the light is scattered at a frequency and the difference in frequency between the incident and the scattered light provides the information of the lattice vibrations. Raman vibrational spectra provides a structural finger print for the molecular identification. Raman spectra of the molecules are widely influenced by the microstructural changes and impurities in the molecule. Hence, the information from Raman spectra like band position, shift, and intensity can be used to characterize the defects and subsurface damages induced by crystal growth. Raman spectra obtained from the instrument gives information on molecular vibrations and crystal structure of materials [70]. Raman spectra of the crystalline molecules satisfy energy conservation rule, wave vector conservation, and polarization selection rule. But if there are some defects and damages in the crystal, this will induce a reduction in symmetry and breakdown of the selection rules will take place. This may give rise to fresh bands in the Raman spectra or broadening of spectra. A change in the Raman spectra is observed when a material is strained or crystal structure of the material is changed [71].

Raman peaks are widely used to identify different crystalline phases of zirconia.

Figure 16 shows the Raman spectra of monoclinic (A), tetragonal (B), and partly tetragonal partly monoclinic (C) zirconia phases. The two sharp peaks at 142 cm⁻¹ and 256 cm-¹ corresponds to tetragonal phase. Another two bands at around 316 cm⁻¹ and 460 cm⁻¹ also correspond to tetragonal phase. The monoclinic phase shows two sharp peaks at 178 cm⁻¹ and 190 cm⁻¹. Broadband at 384 cm-¹ also corresponds to monoclinic structure. Partially transformed zirconia of an overlap of bands corresponding to tetragonal and monoclinic phases are observed. Monoclinic phase content (Vm) of zirconia samples can be evaluated by the following formula which was first proposed by Clarke and Adar [73]:

Figure 16.

Raman spectra of different phases of zirconia [72].

$$\mathrm{Vm}=Im178+\frac{Im189}{0.97\left(\mathrm{It}145+\mathrm{It}260\right)}+Im178+Im189,$$

where Im and It are the intensity of the peaks corresponding to monoclinic and tetragonal phases at different wavenumbers, respectively.

4.7 Scanning electron microscopy (SEM)

Scanning electron microscope is a powerful tool for the two-dimensional topographical imaging. The high-energy electrons emitted from the surface of a material after being exposed to a highly focused beam of electrons from an electron gun are used to produce high-definition images with a resolution of 20 to 0.4 nm [74]. There are two modes electron analysis each one giving different information. Backscattered electrons give contrast based on the different chemical composition across an image. Secondary electrons which are emitted close to the surface of the sample give information on surface topography. Most of the SEM instruments are equipped with energy-dispersive X-ray spectroscopy which is based on the characteristic of X-rays emitted (which is unique to each element) by the element when exposed to an electron beam. A semiquantitative data of chemical composition of the sample is obtained by this method. Surface morphology of zirconia synthesized by different methodologies has different characteristics which are ascertained by SEM data. Figure 17 shows the SEM data of zirconia with EDS spectra. Peaks in EDS spectra give qualitative data of the material synthesized. Energy-dispersive X-ray spectroscopy (EDS) spectra of pure zirconia show the peaks corresponding to Zr and oxygen. Presence of other peaks can be attributed to dopants, impurities, and incompletely removed reagents. Figure 18 shows the agglomerated zirconium oxide powder. Figure 19 shows SEM data of zirconia before and after ball milling at two different magnifications. Surface characteristics of the two are drastically different at 50-μm magnification. However, magnification at 300 nm shows essentially same surface characteristics. It is inferred from this data that morphology of zirconia remains unaffected even after ball milling.

Figure 17.

SEM image and EDX spectra of freshly synthesized zirconia [75].

Figure 18.

SEM picture of agglomerated zirconium oxide powder [76].

Figure 19.

SEM picture of zirconia before (a) and after (b) ball milling [77].

4.8 Transmission electron microscopy (TEM)

Transmission electron microscopy data provides topographical and morphological information about specimens using energetic beam of electrons [78]. When a high-energy electron beam is allowed to pass through a thin slice of a material, the electrons interact with atoms of the material. TEM offers a powerful magnification of the order of a million times. A resolution up to 0.05 nm can be achieved by TEM. The highly detailed images provide valuable insight into elemental and compound structure, leading to provide information on surface features, shape, size, and structure. TEM offers valuable information on the inner structure of the sample. Figure 20 shows the TEM data of nanocrystalline zirconia synthesized by sol–gel method. Figure 21 is the in situ TEM data of zirconia under heavy ion irradiation. Crystalline materials can be made amorphous by ion irradiation. Appearance of voids and phase transformation is observed in the TEM data after ion irradiation.

Figure 20.

TEM image of zirconia nanoparticle synthesized by sol–gel method [79].

Figure 21.

In situ TEM images of ZrO₂ before and after ion irradiation [80].

4.9 Atomic force microscopy (AFM)

Atomic force microscopy is a powerful tool to characterize the surface of a material down to atomic scale. AFM can be used to obtain nanoscale chemical, mechanical, electrical, and magnetic properties. AFM offers the three-dimensional visualization of individual particles and group of particles. This is a very good tool to elucidate the surface roughness and grain size and shape of zirconia nanoparticles and ceramics. Figure 22 is an AFM image that shows shows the size of three-dimensional arrangement of zirconia nanoparticles [81].

Figure 22.

Particle size and distribution zirconia nanoparticles by AFM [81].

Tetragonal zirconia used in dental implants are coated with resin. Adhesiveness of the resin to zirconia increases with roughness of ceramic surface. Surface roughness of zirconia ceramics can be increased by either physical abrasion or chemical etching with hydrogen fluoride (HF). AFM pictures of zirconium ceramic material after different surface treatments are given in Figure 23 [82]. AFM pictures shows changes in roughness with different treatments like (a) abrasion with alumina, (b) etching with 10%HF for 5 minutes, (c) etching with 10%HF for 30 minutes, (d) etching with 20%HF for 5 minutes, (e) etching with 20%HF for 30 minutes, (f) etching with 30%HF for 5 minutes, and (g) etching with 30%HF for 30 minutes. Etching with 30%HF for 30 minutes gave the roughness required for proper adhesiveness of resin to ceramic. SEM images complement AFM data (Figure 24). Data obtained by AFM images are supported by the SEM data. SEM images shows the formation of micro and nanopores on the surface by different treatments.

Figure 23.

AFM images of zirconia ceramics after different surface treatments. (a) APA, (b) 10F5, (c) 10F30, (d) 20F5, (e) 20F30, (f) 30F5, and (g) 30F30.

Figure 24.

SEM images of zirconia ceramics after different surface treatments. (a) APA, (b) 10F5, (c) 10F30, (d) 20F5, (e) 20F30, (f) 30F5, and (g) 30F30.

5. Conclusion

The main source of zirconia, the widely used material in the field of ceramics, electronics, and refractories is zircon, an accessory mineral of placer deposits. Alkali fusion and plasma thermal dissociation are the important decomposition methods adopted widely to convert zircon to zirconia. Various methodologies are adopted for the synthesis of zirconia from its zirconium precursors depending on the particle size and crystal structure. Solution combustion synthesis, sol–gel synthesis, hydrothermal synthesis, and solid-phase sintering are some of the methods adopted for synthesis of different types of zirconia material. Depending on the utility, the synthesized zirconia further undergoes surface modification techniques like surface cleaning and roughening. Doping of zirconia with other materials for increasing the mechanical strength, electrical conductivity, and biocompatibility have been carried out during or after the synthesis. Characterization of synthesized material is essential for ascertaining properties of the synthesized material. Elemental characterization of zirconia can be best accomplished by the destructive solution techniques such as ICP-AES and ICP-MS or by nondestructive solution techniques such as LA-ICP-MS. XPS also can be used for elemental characterization. However, the data obtained pertain only to the surface, but XPS core data can also be used to check the surface chemical state of the zirconium and oxygen. XPS UVB data gives some idea about the crystallinity of the material. Different crystal structure of the synthesized zirconia can be ascertained using XRD data which can be complimented with Raman studies. FTIR data are used to analyze the sample for different functional groups and bonding in the sample. SEM, TEM, and AFM data are used to ascertain the morphology and topography of the prepared material. No single technique is panacea for material characterization, and it is not easy to characterize a material using all the techniques. Hence, judicial choice of combination of techniques for characterization and sufficient knowledge to interpret the data obtained are essential for efficient characterization of a freshly synthesized material.