**2. Experimental**

#### **2.1 Stable suspension**

In order to prepare a stable suspension of the any ceramic, it is desirable to have nano grain sized powders, hence the precursor for MgTiO3, which contentions excess of MgO (Mg2TiO4), prepared by Solar Physical Vapor Deposition (SPVD) was used. The details of the SPVD process could be found elsewhere [15]. The main advantage of SPVD is direct synthesis of nanopowders without any contamination. Moreover, the process is eco-friendly since it uses solar energy which is available abundantly in nature. The KNNT powder required for the EPD was prepared by conventional solid state reaction technique [16, 17]. The KNNT and Mg2TiO4 powders were thoroughly mixed in isopropyl alcohol and acetone, respectively, followed by ultra-sonication and rigorous magnetic stirring. Ultra sonication and magnetic stirring was performed to get homogeneous and well dispersed suspensions.

In both the cases, Triethanolamine (TEA), which is an organic base, was added as the dispersant to increase the zeta potential and colloidal stability of particles in the suspension. It is known that H+ TEA species generated by proton capturing from the isopropyl alcohol/acetone are chemically adsorbed on the ceramic nanoparticles via

**57**

**3. XRD analysis**

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization*

strong hydrogen bonding thereby enhancing their zeta potential and hence the colloidal stability. Also, the addition of TEA is resulted in increase of pH (7 to 9 for KNNT and 6 to 8.2 for Mg2TiO4). The charging mechanism of KNNT in TEA added isopropyl alcohol will be discussed in the coming section. Care is taken while adding TEA since

*(a) Schematic representation of customized EPD cell (b) optical micrograph of MgTiO3 thick film [17], and* 

The EPD was performed at room temperature using a customized EPD setup/cell (**Figure 1a**) and a voltage source (Keithely 2636 B). Pre-cleaned platinum (Pt) plates and foils are used as the electrodes. One of the peculiarities of EPD is that electrodes with various shapes and dimension can be used for deposition. The distance between the two working electrodes was kept constant at 2 cm. The deposition of KNNT was carried out at 100 V for 10 min and Mg2TiO4 at 60 V for 1 min. The picture of thick films obtained is shown in **Figure 1**. The deposition of KNNT as well as MgTiO3 particles happened in the cathode, indicating the negative charge of particles in the suspension. The deposition of particle at cathode is known as cathodic deposition. After deposition the KNNT films were annealed in an oven at 200°C for 1 h and Mg2TiO4 films were dried in desiccators to evaporate the solvent. Then the KNNT films are sintered at 1000°C for 2 hr. For better compaction the dried Mg2TiO4 films were isostatically pressed at 150 MPa for 5 minutes before carrying out the sintering

X-Ray diffraction technique was used for the phase analysis of the films and the microstructures were characterized using a scanning electron microscope (SEM). Later dielectric measurements were carried-out on all the thick films for which

the excess amounts of TEA may lead to the sedimentation of the particle [18].

process in air at 800, 900 and 1000°C for 15 minutes in a box furnace.

metallic top electrodes were sputter deposited through shadow mask.

XRD patterns of KNNT powder, green and sintered KNNT thick films are shown in **Figure 2a** and the XRD patterns of the SPVD obtained MgTiO3

*DOI: http://dx.doi.org/10.5772/intechopen.89996*

**2.2 Electrophoretic deposition**

**Figure 1.**

*(c) KNNT thick film.*

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization DOI: http://dx.doi.org/10.5772/intechopen.89996*

#### **Figure 1.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

we have included in details the preparation of stable suspensions of KNNT and MgTiO3. EPD comprises of two steps; the first step includes the preparation of a stable suspension of the material to be deposited and movement of the particle towards the oppositely charged electrode and the second step involves the deposition of the particle on the substrate surface to forms a film. After the first two deposition steps, a heat-treatment step is normally needed to further densify the deposited films and to eliminate porosity in the film [3, 4]. Compared to other advanced shaping technique, EPD offers easy control of the thickness and morphology of a deposited film through simple adjustment of the deposition time and applied potential [5]. Considering the environmental and health risks of lead-based materials, leadbased materials in the electronics industry are now being replaced with lead-free material. In lead-free ceramic materials, potassium sodium niobate-based materials have received much attention due to its superior piezoelectric properties and high Curie temperature [6]. A piezoelectric vale of 80pC/N has been reported for pure KNN ceramics prepared by solid state reaction route method [7]. However compared to the pure KNN material, doped materials have got high piezoelectric as well as ferroelectric properties. So far, studies have been carried out to improve the piezoelectric properties of KNN ceramics by partial substitutions of Li at Asite and/or Ta at B-site, [8, 9]. Tantalum doping in the B site has a major effect on the materials properties and a piezoelectric value of 205pC/N has been reported for (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) ceramics [10]. Even though the bulk ceramics such as KNNT and MgTiO3 based materials have good electronic properties [10], miniaturizations of electronic devices demand smaller and lighter components with properties comparable to that of bulk material. Potassium Sodium Niobate (KNN) based layers are being employed in applications such as high frequency transducer and in ultra sound imaging [11, 12] and magnesium titanate (MgTiO3) based materials are used in many applications such as high density capacitors, resonators and filters, in wireless communication systems, global positioning system, antennas for communications, radar and broadcasting satellite, operating at micro-wave and millimeter-wave frequencies [13]. The versatile use MgTiO3 is due to its high quality factor (Qf = 160,000 GHz, Qf = 1/tanδ), low dielectric constant (εr ~ 17) and negative temperature coefficient of resonant frequency (τf = − 50 ppm/°C) [14].

In order to prepare a stable suspension of the any ceramic, it is desirable to have

In both the cases, Triethanolamine (TEA), which is an organic base, was added as the dispersant to increase the zeta potential and colloidal stability of particles in the

isopropyl alcohol/acetone are chemically adsorbed on the ceramic nanoparticles via

TEA species generated by proton capturing from the

nano grain sized powders, hence the precursor for MgTiO3, which contentions excess of MgO (Mg2TiO4), prepared by Solar Physical Vapor Deposition (SPVD) was used. The details of the SPVD process could be found elsewhere [15]. The main advantage of SPVD is direct synthesis of nanopowders without any contamination. Moreover, the process is eco-friendly since it uses solar energy which is available abundantly in nature. The KNNT powder required for the EPD was prepared by conventional solid state reaction technique [16, 17]. The KNNT and Mg2TiO4 powders were thoroughly mixed in isopropyl alcohol and acetone, respectively, followed by ultra-sonication and rigorous magnetic stirring. Ultra sonication and magnetic stirring was performed to get homogeneous and well dispersed suspensions.

**56**

**2. Experimental**

**2.1 Stable suspension**

suspension. It is known that H+

*(a) Schematic representation of customized EPD cell (b) optical micrograph of MgTiO3 thick film [17], and (c) KNNT thick film.*

strong hydrogen bonding thereby enhancing their zeta potential and hence the colloidal stability. Also, the addition of TEA is resulted in increase of pH (7 to 9 for KNNT and 6 to 8.2 for Mg2TiO4). The charging mechanism of KNNT in TEA added isopropyl alcohol will be discussed in the coming section. Care is taken while adding TEA since the excess amounts of TEA may lead to the sedimentation of the particle [18].

#### **2.2 Electrophoretic deposition**

The EPD was performed at room temperature using a customized EPD setup/cell (**Figure 1a**) and a voltage source (Keithely 2636 B). Pre-cleaned platinum (Pt) plates and foils are used as the electrodes. One of the peculiarities of EPD is that electrodes with various shapes and dimension can be used for deposition. The distance between the two working electrodes was kept constant at 2 cm. The deposition of KNNT was carried out at 100 V for 10 min and Mg2TiO4 at 60 V for 1 min. The picture of thick films obtained is shown in **Figure 1**. The deposition of KNNT as well as MgTiO3 particles happened in the cathode, indicating the negative charge of particles in the suspension. The deposition of particle at cathode is known as cathodic deposition. After deposition the KNNT films were annealed in an oven at 200°C for 1 h and Mg2TiO4 films were dried in desiccators to evaporate the solvent. Then the KNNT films are sintered at 1000°C for 2 hr. For better compaction the dried Mg2TiO4 films were isostatically pressed at 150 MPa for 5 minutes before carrying out the sintering process in air at 800, 900 and 1000°C for 15 minutes in a box furnace.

X-Ray diffraction technique was used for the phase analysis of the films and the microstructures were characterized using a scanning electron microscope (SEM). Later dielectric measurements were carried-out on all the thick films for which metallic top electrodes were sputter deposited through shadow mask.

## **3. XRD analysis**

XRD patterns of KNNT powder, green and sintered KNNT thick films are shown in **Figure 2a** and the XRD patterns of the SPVD obtained MgTiO3

#### **Figure 2.**

*(a) XRD patterns of KNNT powder, green film and sintered thick films (b) XRD patterns of Mg2TiO4 nanopowder, EPD films sintered at 800 (15 min) and 900°C (15 min) [17].*

nanoparticle and the thick films sintered at different temperatures is shown in **Figure 2b**. XRD analysis of KNNT revealed that the powder as well as the films has perovskite monoclinic structure (JCPDS 770038) along with some secondary phase. On comparing the patterns of green and sintered films, no additional peaks were observed, indicating that the quality of the film merely depend on the quality of the powder. However, in the case of Mg2TiO4, a phase change between the SPVD nanopowders and the sintered films was observed. An apparent decrease in the peak width between the nanopowders and the sintered films as also observed in Mg2TiO4samples, which indicate an increase in the crystallite size of the sintered films [19]. Even though the starting powder was Mg2TiO4, a pure MgTiO3 phase without secondary phases was obtained after sintering because nanopowders were very fine and very reactive. Moreover, the XRD patterns were matched to crystallographic data on synthetic geikielite crystalline phase and the peaks were indexed to a rhombohedral unit cell with a = b = 0.5057(2) nm and c = 1.3890(4) nm [17].

### **4. Charging mechanism and deposition**

Depending on the surface charge on the particle in a suspension, the deposition can be anodic or cathodic deposition. The deposition of a negatively charged particle on anode is called anodic electrophoretic deposition and the deposition of a positively charged particle on cathode is called cathodic electrophoretic deposition. The surface charge of the particle can be easily modified by the appropriate choice of solvent or/and surfactant [18, 20]. Moreover, different surface charge can be introduced on KNN particle using a single solvent with different surfactants [21].

Here we have observed that the KNNT particles were deposited on cathode yielding a smooth deposit. Cathodic deposition of KNNT particle indicates that KNNT particles have acquired positive charge in the medium. Since TEA is a weak base, it increases the pH of the suspension and when it is added to isopropyl alcohol it takes the proton from isopropyl alcohol and acquires positive charge. The overall reaction can be written as [18].

**59**

NH<sup>+</sup>

**Figure 3.**

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization*

C3H7OH + N(C2H4OH)3 ↔ C3H7O<sup>−</sup> + NH+

**5. Effect of EPD voltage and time on the deposition weight**

KNNT particles confer positive charge to them and the electrostatic repulsion force between TEA adsorbed KNNT particles prevent the agglomeration of the particles in the medium. Upon applying an electric field of proper strength, the charged particles get attracted towards oppositely charged plate and there it get neutralized and the deposition of KNNT particles occurs on the plate leaving

*Schematic representation of charge acquisition of KNNT particle in TEA added isopropyl alcohol medium.*

(C2H4OH)3 back to the solution. Even though the stabilization of suspension is via electrostatic repulsion force, the high molecular weight TEA may also contribute to some steric stabilization mechanism [18]. The green and sintered film showed good adhesion with the Pt-electrode and showed no micro cracks,

According to Hamaker's equation the yield of electrophoretic deposition (w) is related to electrophoretic mobility (μ), surface area of the electrode (A), electric field strength (E) particle mass concentration in the suspension (C) and deposition

∫

**Figure 4** shows the variation of weight of KNNT particles deposited on substrate at various dc voltages and the variation in deposit weight with deposition times at a particular voltage (100 V). From **Figure 4**, it is evident that the rate of deposit increases with the increase in applied voltage and deposition time.

*t*2

*t*1 (C2H4OH)3 (1)

(C2H4OH)3 get adsorbed on the surface of the

*μ*. *E*. *A*.*C*.*dt* (2)

*DOI: http://dx.doi.org/10.5772/intechopen.89996*

This charged species NH<sup>+</sup>

peelings, or pores (**Figure 3**).

time (t) through the following relation [22].

*w* =

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization DOI: http://dx.doi.org/10.5772/intechopen.89996*

**Figure 3.** *Schematic representation of charge acquisition of KNNT particle in TEA added isopropyl alcohol medium.*

$$\rm C\_3H\_7OH + N(C\_2H\_4OH)\_3 \leftrightarrow C\_3H\_7O^- + NH^+\\(C\_2H\_4OH)\_3 \tag{1}$$

This charged species NH<sup>+</sup> (C2H4OH)3 get adsorbed on the surface of the KNNT particles confer positive charge to them and the electrostatic repulsion force between TEA adsorbed KNNT particles prevent the agglomeration of the particles in the medium. Upon applying an electric field of proper strength, the charged particles get attracted towards oppositely charged plate and there it get neutralized and the deposition of KNNT particles occurs on the plate leaving NH<sup>+</sup> (C2H4OH)3 back to the solution. Even though the stabilization of suspension is via electrostatic repulsion force, the high molecular weight TEA may also contribute to some steric stabilization mechanism [18]. The green and sintered film showed good adhesion with the Pt-electrode and showed no micro cracks, peelings, or pores (**Figure 3**).

### **5. Effect of EPD voltage and time on the deposition weight**

According to Hamaker's equation the yield of electrophoretic deposition (w) is related to electrophoretic mobility (μ), surface area of the electrode (A), electric field strength (E) particle mass concentration in the suspension (C) and deposition time (t) through the following relation [22].

$$
\mu = \int\_{t\_1}^{t\_2} \mu.E.A. \,\mathrm{C}dt\tag{2}
$$

**Figure 4** shows the variation of weight of KNNT particles deposited on substrate at various dc voltages and the variation in deposit weight with deposition times at a particular voltage (100 V). From **Figure 4**, it is evident that the rate of deposit increases with the increase in applied voltage and deposition time.

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

nanoparticle and the thick films sintered at different temperatures is shown in **Figure 2b**. XRD analysis of KNNT revealed that the powder as well as the films has perovskite monoclinic structure (JCPDS 770038) along with some secondary phase. On comparing the patterns of green and sintered films, no additional peaks were observed, indicating that the quality of the film merely depend on the quality of the powder. However, in the case of Mg2TiO4, a phase change between the SPVD nanopowders and the sintered films was observed. An apparent decrease in the peak width between the nanopowders and the sintered films as also observed in Mg2TiO4samples, which indicate an increase in the crystallite size of the sintered films [19]. Even though the starting powder was Mg2TiO4, a pure MgTiO3 phase without secondary phases was obtained after sintering because nanopowders were very fine and very reactive. Moreover, the XRD patterns were matched to crystallographic data on synthetic geikielite crystalline phase and the peaks were indexed to a rhombohedral unit cell with a = b = 0.5057(2) nm and

*(a) XRD patterns of KNNT powder, green film and sintered thick films (b) XRD patterns of Mg2TiO4*

*nanopowder, EPD films sintered at 800 (15 min) and 900°C (15 min) [17].*

Depending on the surface charge on the particle in a suspension, the deposition can be anodic or cathodic deposition. The deposition of a negatively charged particle on anode is called anodic electrophoretic deposition and the deposition of a positively charged particle on cathode is called cathodic electrophoretic deposition. The surface charge of the particle can be easily modified by the appropriate choice of solvent or/and surfactant [18, 20]. Moreover, different surface charge can be introduced on KNN particle using a single solvent with

Here we have observed that the KNNT particles were deposited on cathode yielding a smooth deposit. Cathodic deposition of KNNT particle indicates that KNNT particles have acquired positive charge in the medium. Since TEA is a weak base, it increases the pH of the suspension and when it is added to isopropyl alcohol it takes the proton from isopropyl alcohol and acquires positive charge. The overall

**58**

c = 1.3890(4) nm [17].

**Figure 2.**

different surfactants [21].

reaction can be written as [18].

**4. Charging mechanism and deposition**

#### **Figure 4.**

*Amount of KNNT particles deposited on substrate at various dc voltages and various deposition time (at constant voltage of 100 V).*

At high voltage and longer deposition durations, though the rate of deposition was high, it does not yield smooth film surface. The surface morphology was very rough and the films tend to peel off from the substrate due to the low adhesion between the film and the Pt substrate. At low voltages and short deposition time, the amount of deposit was too small to form a uniformly thick film. Since we obtained films with good surface morphology at 100 V, this film was used for further studies.

### **6. SEM analysis**

When there is dominancy of gravity and/or adhesive force of suspension over the inter particle van der Waals force, dislodgement of particle may occur during the lifting of the substrate out of suspension. This may lead to crack formation or micro cavities on the film surface. Hence, to study the quality of the deposited film, SEM images were taken and analyzed. The SEM images of KNNT thick film sintered at 1000°C are shown in **Figure 5a**. From **Figure 5a**, it is evident that KNNT particles are well deposited on the substrate with good compaction. In addition, no cracks were observed on the surface indicating that the processing conditions are favorable to get crack free thick films. Thickness as well the uniformity in thickness was analyzed from SEM images using Image J software and it was found that the film had uniform thickness of ̴32 μm over the entire deposited region.

SEM micrographs MgTiO3 thick film is shown in **Figure 5c**–**f**. A regular shaped grain with an average size of 50 nm can be observed in as deposited green films (**Figure 5c**). Whereas a porous microstructure with an average grain size of 60–70 nm was observed in MgTiO3 thick films sintered at 800°C (**Figure 5d**). It is clear from **Figure 5e** that the films sintered at 900°C have more homogeneous and denser microstructure with higher grain size (100 nm). However, at higher sintering temperature (1000°C) inhomogeneous grain growth and delaminating effects were observed. The cross-sectional (CS) image of film sintered at 900°C was taken to find the thickness (inset of **Figure 5c**) and the thickness was about 18–20 μm.

**61**

**Figure 5.**

**7. Dielectric constant (εr)**

*regions are the film and the lighter area is the Pt substrate).*

Variation of dielectric constant and dielectric loss with frequency at room temperature for sintered KNNT thick film is obtained using a metal- insulatormetal structure and the data is shown in **Figure 6**. Dielectric constant value of 320 (at 1 MHz) was obtained for sintered KNNT thick film. The dielectric value of the KNNT film (320) at 1 MHz was lower than the value reported for KNN thick films fabricated via electrophoretic deposition in acetone (393) [21] and aqueous

*SEM images (a) surface morphology of the sintered KNNT thick film (b) SEM showing the thickness of the KNNT film sintered at 1000°*C. SEM *micrograph of (c) as-deposited green film. (d–f) SEM images of MgTiO3 thick films sintered at (b) 800°C (15 min), (c) 900°C (15 min); the inset shows the cross sectional SEM image of the film sintered at 900°C and (d) 1000°C (15 min) sintered film showing delamination (the dark island like* 

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization*

*DOI: http://dx.doi.org/10.5772/intechopen.89996*

*Fabrication of Dielectric Thick Films by Electrophoretic Deposition and Their Characterization DOI: http://dx.doi.org/10.5772/intechopen.89996*

#### **Figure 5.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

At high voltage and longer deposition durations, though the rate of deposition was high, it does not yield smooth film surface. The surface morphology was very rough and the films tend to peel off from the substrate due to the low adhesion between the film and the Pt substrate. At low voltages and short deposition time, the amount of deposit was too small to form a uniformly thick film. Since we obtained films with good surface morphology at 100 V, this film was used for

*Amount of KNNT particles deposited on substrate at various dc voltages and various deposition time* 

When there is dominancy of gravity and/or adhesive force of suspension over the inter particle van der Waals force, dislodgement of particle may occur during the lifting of the substrate out of suspension. This may lead to crack formation or micro cavities on the film surface. Hence, to study the quality of the deposited film, SEM images were taken and analyzed. The SEM images of KNNT thick film sintered at 1000°C are shown in **Figure 5a**. From **Figure 5a**, it is evident that KNNT particles are well deposited on the substrate with good compaction. In addition, no cracks were observed on the surface indicating that the processing conditions are favorable to get crack free thick films. Thickness as well the uniformity in thickness was analyzed from SEM images using Image J software and it was found that the film

SEM micrographs MgTiO3 thick film is shown in **Figure 5c**–**f**. A regular shaped grain with an average size of 50 nm can be observed in as deposited green films (**Figure 5c**). Whereas a porous microstructure with an average grain size of 60–70 nm was observed in MgTiO3 thick films sintered at 800°C (**Figure 5d**). It is clear from **Figure 5e** that the films sintered at 900°C have more homogeneous and denser microstructure with higher grain size (100 nm). However, at higher sintering temperature (1000°C) inhomogeneous grain growth and delaminating effects were observed. The cross-sectional (CS) image of film sintered at 900°C was taken to find the thickness (inset of **Figure 5c**)

had uniform thickness of ̴32 μm over the entire deposited region.

and the thickness was about 18–20 μm.

**60**

further studies.

*(at constant voltage of 100 V).*

**Figure 4.**

**6. SEM analysis**

*SEM images (a) surface morphology of the sintered KNNT thick film (b) SEM showing the thickness of the KNNT film sintered at 1000°*C. SEM *micrograph of (c) as-deposited green film. (d–f) SEM images of MgTiO3 thick films sintered at (b) 800°C (15 min), (c) 900°C (15 min); the inset shows the cross sectional SEM image of the film sintered at 900°C and (d) 1000°C (15 min) sintered film showing delamination (the dark island like regions are the film and the lighter area is the Pt substrate).*
