**1. Introduction**

300 Advances in Crystallization Processes

Whittaker, V.P. (1977). The electromotor system of Torpedo as a model cholinergic system.

Zirconium titanate (ZT)-based ceramic materials, ZrTiO4, have many attractive properties: high resistivity, high dielectric constant, high permittivity at microwave frequencies and excellent temperature stability for microwave properties (Bianco *et al*, 1999; Leoni *et al*, 2001). These materials have an extremely wide range of technological application, such as in microwave telecommunications (as capacitors, dielectric resonators in filters, and oscillators) (Navio *et al*, 1993; Navio *et al*, 1994; Azough *et al* 1996), in the manufacture of hightemperature pigments (Hund and Anorg, 1985; Dondi *et al*, 2006) in catalysis (as effective acid-base bifunctional catalysts and photocatalysts) (Tanabe, 1970; Araka and Tanabe, 1980), as structural ceramics (Parker, 1990) and, more recently, as biomaterial coated on 316L SS for biomedical applications (Devi *et al*, 2011).

The formation of ZrTiO4 has been studied by several authors (McHale *et al*, 1989; Christoffersen *et al*, 1992a; Bhattacharya *et al*, 1996; Hom *et al*, 2001; Ananta *et al*, 2003; Kim *et al*, 2004; Troitzsch *et al*, 2005; Vittayakorn *et al*, 2006; Licina *et al*, 2008).

Zirconium titanate is normally synthesized by solid-state reaction of oxide mixtures between ZrO2 and TiO2 at elevated temperatures (1200-1600°C) and long heating times and also requires post treatment such as energy-intensive grinding/milling procedures for powder formation and this process usually leads to inhomogeneous, coarse, and multiphase powders of poor purity (Lynch, 1972; Swartz, 1982; McHale *et al*, 1983; McHale *et al,* 1986; Parker, 1990; Christoffersen *et al*, 1992b; Park et al, 1996; Stubicar *et al*, 2001; Troitzsch *et al,* 2004; Dondi *et al,*  2006). ZrTiO4 ceramics have the orthorhombic structure of α-PbO2 (Blasse, 1966; Newnham, 1967) and belongs to the space group Pbcn. Above 1200 °C the Zr ion and Ti ion of the high temperature normal phase are randomly distributed on octahedral site in α-PbO2 type structure. (McHale *et al*, 1983; Christoffersen *et al*, 1992b; Azough *et al*, 1993; Ul'yanova *et al*, 1995). In contrast to the displaced transitions which take place at discrete temperature, ZrTiO4 ceramics undergoes a continuous phase transition of the normal to incommensurate over the temperatures of 1200°C to 1100°C by increasing order in the Zr-Ti distributions.

The chemical preparation of reactive precursors offers advantages over traditional processing techniques because of the higher purity and better homogeneity obtained the

Synthesis and Characterization of Crystalline Zirconium Titanate Obtained by Sol-Gel 303

Actually the sol-gel processes were developed both in academic research and in industry, producing glasses by straightforward polymerization of molecular precursors in solution. Basically, sol-gel process carried out in a liquid medium. This process involves the evolution of inorganic networks through the formation of a colloidal suspension which is called sol and gelation of the sol to form a network in a continuous liquid phase which is denoted as gel. Three reactions generally describe the sol-gel process: (1) hydrolysis reaction, (2) alcohol condensation process, and (3) water condensation process. The sol-gel approach also provides an alternative and usual way for synthesis of nanomaterials. Combined with chemical nanotechnologies, remarkable progress has been achieved and sol-gel techniques have taken their place as a fundamental approach to the development of new nanomaterials. The sol-gel synthesis method has been used for the production of metal, metal oxide and ceramic nanoparticles with high purity and good homogeneity. If an organic surfactant is added to the sol as the structuring agent, it is even possible to obtain an ordered porous structure in two dimensions or three dimensions. The sol-gel process is considered as a lowtemperature synthesis method that gives pure, homogeneous nanoparticles with good size distribution in the design of complex nanoarchitectures. Furthermore, many kinds of nanoparticles including oxides, suldes, metals, and semiconductors with nanoporous structures can be synthesized through a precise heat treatment. The versatility of the process is largely due to the rich and varied chemistry of organometallic precursors, combined with

Samples containing multi-phases are important from the technological perspective and are strongly superposed (Sham *et al*, 1998). Consentino *et al.* prepared ceramic powders from the mixture of zirconium oxychloride and titanium chloride in stoichiometric quantities in the presence of citric acid (60 °C) and ethylene glycol. By using this technique, the authors observed that after treated at 600 ºC for 1 hour still amorphous. In 730 °C, had the crystalline phase of orthorhombic ZrTiO4, contrary to reported by Karakchiev *et al* which obtained zirconium titanate by sols hydrated in 1:1 ratio of Zr:Ti with the presence of TiO2 as anatase, below 600 °C. At 600 °C this form disappears and gives way to the ZrTiO4 peaks. The preparation of ZrTiO4 and Zr0.8Sn0.2TiO4 by pulsed laser deposition was reported by Viticoli *et al*. Films of ZrTiO4 were prepared, deposited between 450 and 550 °C. At 450 °C, a weak intensity peak at 2θ = 30.48° indicates the presence of crystalline zirconium titanate, peaks at 2θ = 32-35° also suggest the presence of phases for Ti2O3, TiO2 and ZrO2. Raising the temperature to 550 °C the intensity of reflections (1 1 1) characteristic of ZrTiO4 and the reflections (0 2 0), (2 0 0) and (2 2 2) at 2θ = 32.6°, 35.7° and 63.3 can be identified. Under these conditions only the contributions of ZrTiO4 can be observed, indicating the formation of a single crystalline phase of ZrTiO4. The crystallographic structure for the films deposited, containing tin presents an intense peak at 450°C at 2θ = 33.06° associated with reflection (1 0 4) of Ti2O3. Peaks at 2θ = 32-35° suggest the presence of phases for SnO2, TiO2 and ZrO2, while a weak intensity peak indicates the crystallization of Zr0.8Sn0.2TiO4 (0 0 2). (Bhattacharya *et* 

In this study, ZT powders were obtained from hydrolysis reactions of zirconium n-propoxide (NPZ) and titanium isopropoxide (tetra-isopropyl titanate – TPT). These reactions were performed by hydrolyzing the alkoxides separately, at the molar ratio of Zr:Ti of 1:1, in n-propanol, using the sol-gel process at ambient temperature (20°C) and

the low processing temperature (Qiao *et al*, 2011).

*al*, 1996; Troitzsch *et al*, 2005).

lower processing temperatures and improved material properties. Table 1 summarizes the procedures currently used to prepare pure ZrTiO4 materials by non-conventional or chemical routes (Navio et al, 1992a; Pol *et al*, 2007).


Table 1. Wet-chemical routes for synthesizing ZrTiO4 materials

lower processing temperatures and improved material properties. Table 1 summarizes the procedures currently used to prepare pure ZrTiO4 materials by non-conventional or

Precursors Preparation procedure References

2.Calcined precipitates shaped into pellets, pressed at 100 MPa, sintered at 1600 °C for 10 h and posttreated at 1500 °C for 5 h, quenched to roomtemperature air and annealed at 700 °C for 3days

1. Using 2-propanol as solvent, mixing for 5 h at 82 °C, hydrolysis with H20 at room temperature

2. Solid filtered, washed with hot water and dried

Classical hydrolytic sol-gel, where metal alcoxides are dissolved in alcohol, after o gel is dried and

Ikawa *et al*, 1988; Ikawa *et al*, 1991

Yamagushi *et al*, 1989

Muñoz *et al*, 1990

Hirano *et al*, 1991; Komarneni *et al*, 1999; Karakchiev *et al*, 2001

Navio *et al*, 1992b

Gavrilov *et al*, 1996

Bianco *et al*, 1998

Leoni *et al*, 2001

1.Coprecipitation of mixed oxides using

concentrated ammonia water

and increased to 75 °C

calcined.

Zirconyl chloride 1. Mixing and precipitation with methanol,

ammonia solution at pH 8-9

TiCl4 and ZrOCl2 Chemical precipitation using ammonia, produced

ZT nanopowders

Table 1. Wet-chemical routes for synthesizing ZrTiO4 materials

at 120 °C under reduced pressure

Partial and controlled hydrolysis using CH3COOH-propanol, and a ratio h = [CH3COOH]/[metal] 1 ≤ h ≤ 8

If h ≤ 3, homogeneous gels were obtained If h > 3, white precipitate M(OR)x(CH3COO)y

hydrogen peroxide solution and aqueous

Mixing with ethylene glycol, Ti(OBut)4, ZrOC12.8H20 and citric acid, at 110-120 °C

2. The gelled mass was washed with acetone, filtered and dried slowly for several days

Process involves hydrolysis of the starting salts, nucleation of zirconia and titania, nucleus growth, and the formation of ZrTiO4 above 1150 °C.

Sol-gel process produced long ZrTiO4 fibers Lu *et al*, 2003

chemical routes (Navio et al, 1992a; Pol *et al*, 2007).

Zr(SO4)2, Ti(SO4)2

Zirconium isopropoxide titanium isopropoxide

Zirconium propoxide titanium isopropoxide

Zirconium alcoxide,

Zirconium oxychloride and titanyl chloride

Ti(OBut)4, ZrOC12.8H2O (Polymeric precursor)

Titanium sulfate, ZrOC12.8H2O

titanium alcoxide

Actually the sol-gel processes were developed both in academic research and in industry, producing glasses by straightforward polymerization of molecular precursors in solution. Basically, sol-gel process carried out in a liquid medium. This process involves the evolution of inorganic networks through the formation of a colloidal suspension which is called sol and gelation of the sol to form a network in a continuous liquid phase which is denoted as gel. Three reactions generally describe the sol-gel process: (1) hydrolysis reaction, (2) alcohol condensation process, and (3) water condensation process. The sol-gel approach also provides an alternative and usual way for synthesis of nanomaterials. Combined with chemical nanotechnologies, remarkable progress has been achieved and sol-gel techniques have taken their place as a fundamental approach to the development of new nanomaterials. The sol-gel synthesis method has been used for the production of metal, metal oxide and ceramic nanoparticles with high purity and good homogeneity. If an organic surfactant is added to the sol as the structuring agent, it is even possible to obtain an ordered porous structure in two dimensions or three dimensions. The sol-gel process is considered as a lowtemperature synthesis method that gives pure, homogeneous nanoparticles with good size distribution in the design of complex nanoarchitectures. Furthermore, many kinds of nanoparticles including oxides, suldes, metals, and semiconductors with nanoporous structures can be synthesized through a precise heat treatment. The versatility of the process is largely due to the rich and varied chemistry of organometallic precursors, combined with the low processing temperature (Qiao *et al*, 2011).

Samples containing multi-phases are important from the technological perspective and are strongly superposed (Sham *et al*, 1998). Consentino *et al.* prepared ceramic powders from the mixture of zirconium oxychloride and titanium chloride in stoichiometric quantities in the presence of citric acid (60 °C) and ethylene glycol. By using this technique, the authors observed that after treated at 600 ºC for 1 hour still amorphous. In 730 °C, had the crystalline phase of orthorhombic ZrTiO4, contrary to reported by Karakchiev *et al* which obtained zirconium titanate by sols hydrated in 1:1 ratio of Zr:Ti with the presence of TiO2 as anatase, below 600 °C. At 600 °C this form disappears and gives way to the ZrTiO4 peaks. The preparation of ZrTiO4 and Zr0.8Sn0.2TiO4 by pulsed laser deposition was reported by Viticoli *et al*. Films of ZrTiO4 were prepared, deposited between 450 and 550 °C. At 450 °C, a weak intensity peak at 2θ = 30.48° indicates the presence of crystalline zirconium titanate, peaks at 2θ = 32-35° also suggest the presence of phases for Ti2O3, TiO2 and ZrO2. Raising the temperature to 550 °C the intensity of reflections (1 1 1) characteristic of ZrTiO4 and the reflections (0 2 0), (2 0 0) and (2 2 2) at 2θ = 32.6°, 35.7° and 63.3 can be identified. Under these conditions only the contributions of ZrTiO4 can be observed, indicating the formation of a single crystalline phase of ZrTiO4. The crystallographic structure for the films deposited, containing tin presents an intense peak at 450°C at 2θ = 33.06° associated with reflection (1 0 4) of Ti2O3. Peaks at 2θ = 32-35° suggest the presence of phases for SnO2, TiO2 and ZrO2, while a weak intensity peak indicates the crystallization of Zr0.8Sn0.2TiO4 (0 0 2). (Bhattacharya *et al*, 1996; Troitzsch *et al*, 2005).

In this study, ZT powders were obtained from hydrolysis reactions of zirconium n-propoxide (NPZ) and titanium isopropoxide (tetra-isopropyl titanate – TPT). These reactions were performed by hydrolyzing the alkoxides separately, at the molar ratio of Zr:Ti of 1:1, in n-propanol, using the sol-gel process at ambient temperature (20°C) and

Synthesis and Characterization of Crystalline Zirconium Titanate Obtained by Sol-Gel 305

In order to determine the mineralogical phases by X-Ray Diffraction (XRD), a Philips X-Ray Diffractometer was used (model X'Pert MPD) equipped with a graphite monochromator and rotational anode, operated at 40 kV and 40 mV. The data were collected via Cu-K<sup>α</sup> radiation at a step of 0.01° and time per step of 2 s, in order to determine the phases present

Fourier transform infrared spectroscopy (FT-IR) analysis of dried and annealed powders were carried out in an Impact 400, Nicolet spectrometer in the wavenumber range 400–4000 cm-1 at resolution of 4 cm-1 for studying the chemical groups. For this analysis, KBr pellets were

The particle size distribution of agglomerates and particles of sintered powder was determined by a laser diffraction spectrometer Cilas, model 1180. The detection range of this

Thermogravimetric and thermodifferential analyses are shown in Figures 2a for hydrolysis of alkoxides together and 2b for alkoxide hydrolysis separately. The curves obtained are very similar, initially, with a great loss of mass between approximately 40 °C and 200 ºC, which is due, probably, to dehydration, accompanied by an endothermal peak due to the energy consumed to release volatiles. An exothermal mass can be seen in Figure 2a, at 703 ºC. It is ascribed to crystallization of the orthorhombic phase of zirconium titanate. According to Khairulla and Phule (1992), the peaks by differential thermal analysis (DTA) at 350 and 550 °C are related to the removal of organic compounds and an exothermal event without loss of mass at approximately 710 °C is caused by the formation of the crystalline phase of ZrTiO4. Macan *et al*12, describe that a first loss of mass seen in the TGA and DSC curve as accompanied by an endothermal peak at 83 °C due to the evaporation of adsorbed water. Loss of mass diminishes constantly as temperature increases as a function of the slow degradation of residual organic matter, and the exothermal peak at 703°C is due to the crystallization of amorphous ZrTiO4, confirmed in the preliminary research (Consentino *et* 

In Figure 2b two exothermal peaks were observed, 261°C and 460 °C, accompanied by loss of mass, due to the burning of the remaining organic matter. Bhattacharya *et al* suggest that this inflexion is due to the removal of structural anionic species. As the temperature increases, decomposition reactions occur and a broad exothermic peak at 460 °C. The weight loss continues and stabilizes at about 500 °C. At 703°C no exothermal peak that could be attributed to the crystallization of zirconium titanate was observed. During the cooling process endothermal events occurred, with hardly any loss of mass at 1260, 948 and 897 °C, which may be related to crystalline phase transitions, as described by Park *et al* (1996) in the solid-state reaction of oxide mixtures between ZrO2 and TiO2 at elevated temperatures.

**2.2.2 X-Ray diffraction** 

**2.2.3 Fourier transform infrared spectroscopy (FT-IR)** 

**2.2.4 Particle size distribution by LASER diffraction spectrometry** 

pressed to hold the samples to be analyzed.

equipment is between 0.04 and 2500 μm.

**3. Results and discussion** 

*al*, 2003).

in the samples.

influence nitric acid, in order to verify the form of crystallization of ZrTiO4 under these conditions. The results were compared to the previous work (Santos *et al*, 2010).
