**2. Microwave plasma sintering of nanoparticles**

Considerable research has been carried out to understand the interaction of microwave radiation with materials [33, 34]. The ability of microwaves to penetrate work pieces enables volumetric heating at a rapid rate, thus avoiding the need of external heat sources [33]; this makes it an attractive technique in material processing. However, poor coupling in non-plasma microwave processing may result in a non-uniform heating of the substrates [35]. One of the advantages of microwave plasma treatment is the combined advantage inherent in microwave and plasma heating in terms of volumetric, homogeneous and rapid heating [2, 36, 37]. For example, the work of Twomey et al. [37] demonstrates that homogenous heating of substrates could be achieved using microwave plasma treatments including considerably lower cycle times [38] compared to non-plasma microwave and furnace heating. In this section, the use of microwave plasma processing for the sintering of Nickel oxide (NiO) and Titanium dioxide (TiO<sup>2</sup> ) particles is presented.

previously [47–51]. In this study, a treatment temperature of approximately 450°C was used for a 5-min period for both the furnace and RDS treatments. This treatment time was chosen because it yielded the best photovoltaic performance for RDS treatment. Overall, a cycle time of ~120 min was required for the furnace sintering including heat up, maintenance at the max 450°C temperature and cool down. In contrast, the RDS processing required only 15 min,

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In this Section, the properties of the furnace sintered coatings, such as morphology, crystal structure, dye adsorption, chemical composition and photovoltaic performance are compared

A typical SEM micrograph of the morphology of RDS and furnace treated coatings is given in **Figure 3**. A higher level of interconnectivity with reduced grain growth for the RDS treated coatings when compared with the furnace sintered coatings can be observed. The increased grain growth in the furnace sintered coatings could be due to a slower cooling rate (10°C/min) used in the furnace process. In contrast, the RDS technique requires only a 5-min cooling time

A further examination of NiO coatings using an FIB/SEM cross section micrograph (**Figure 4**) indicated that the RDS treated coating displays an improved level of porosity as well as interfacial connection between the NiO coating and FTO substrate. This could be due to the difference in the type of heating mechanisms in both sintering techniques. RDS treatment encompasses volumetric heating, which delivers more efficient heating inside the NiO coating matrix than that obtained with furnace sintering where a conductive type of heating is

included all stages such as pump down, plasma treatment, and cooling.

with the NiO coatings treated for 5 min, using the RDS technique.

**Figure 2.** (a) 0.6 (b) 2.5 μm thick NiO coatings deposited using spray technique.

*2.1.1.1. Morphological analyses of porous NiO coatings*

following the plasma treatment.

#### **2.1. Sintering of nickel oxide (NiO) nanoparticles**

NiO coatings are used extensively as a photocathode in the construction of *p*-type DSSCs [39–42] due to their *p*-type semiconductivity [43], and well defined electrical and optical properties [44]. Furthermore, its bandgap energy which range from 3.6 to 4.0 eV helps to make it a model semiconductor substrate [43]. Also, it is considered a good electron donor for numerous photo sensitizers due to its valance bond potential; thus, the NiO coatings are readily quenched with many dye sensitizers [45]. In this section, the use of microwave plasma processing for the sintering of NiO particles to produce coatings for use in DSSC electrodes is discussed [42, 46].

#### *2.1.1. Rapid discharge sintering vs. conventional furnace sintering*

In order to process NiO using the RDS technique, a spray technique is used to deposit solvent slurry of NiO nanoparticles onto conductive glass substrates (**Figure 2**). The spray technique involves suspending the NiO nanoparticles in alcohol; the mixture is then applied via spraying onto fluorine-doped tin oxide (FTO) coated glass substrates to form the coating. A subsequent step is carried out after spraying in order to enhance the interconnectivity between the NiO particles by sintering, as well as an increased level of adhesion to the conductive glass substrate. This sintering step was investigated using both microwave plasma and furnace treatments [42, 46]. The latter was carried out using a CWF 1200 Carbolite tube air furnace. Typical furnace sintering temperatures in the range of 300–450°C have been reported Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings http://dx.doi.org/10.5772/intechopen.71686 113

**Figure 2.** (a) 0.6 (b) 2.5 μm thick NiO coatings deposited using spray technique.

The specific focus is on the sintering of nanoparticles of nickel oxide (NiO) and titanium diox-

treatments on thermal sensitive crystalline phase changes, is also discussed. A further application reported is that of the use of microwave plasma treatments for the oxidation of metal

Considerable research has been carried out to understand the interaction of microwave radiation with materials [33, 34]. The ability of microwaves to penetrate work pieces enables volumetric heating at a rapid rate, thus avoiding the need of external heat sources [33]; this makes it an attractive technique in material processing. However, poor coupling in non-plasma microwave processing may result in a non-uniform heating of the substrates [35]. One of the advantages of microwave plasma treatment is the combined advantage inherent in microwave and plasma heating in terms of volumetric, homogeneous and rapid heating [2, 36, 37]. For example, the work of Twomey et al. [37] demonstrates that homogenous heating of substrates could be achieved using microwave plasma treatments including considerably lower cycle times [38] compared to non-plasma microwave and furnace heating. In this section, the use of microwave plasma processing for the sintering of Nickel oxide (NiO) and Titanium

NiO coatings are used extensively as a photocathode in the construction of *p*-type DSSCs [39–42] due to their *p*-type semiconductivity [43], and well defined electrical and optical properties [44]. Furthermore, its bandgap energy which range from 3.6 to 4.0 eV helps to make it a model semiconductor substrate [43]. Also, it is considered a good electron donor for numerous photo sensitizers due to its valance bond potential; thus, the NiO coatings are readily quenched with many dye sensitizers [45]. In this section, the use of microwave plasma processing for the sintering of NiO particles to produce coatings for use in DSSC electrodes

In order to process NiO using the RDS technique, a spray technique is used to deposit solvent slurry of NiO nanoparticles onto conductive glass substrates (**Figure 2**). The spray technique involves suspending the NiO nanoparticles in alcohol; the mixture is then applied via spraying onto fluorine-doped tin oxide (FTO) coated glass substrates to form the coating. A subsequent step is carried out after spraying in order to enhance the interconnectivity between the NiO particles by sintering, as well as an increased level of adhesion to the conductive glass substrate. This sintering step was investigated using both microwave plasma and furnace treatments [42, 46]. The latter was carried out using a CWF 1200 Carbolite tube air furnace. Typical furnace sintering temperatures in the range of 300–450°C have been reported

surfaces, in order to produce porous oxide ceramic (TiO<sup>2</sup>

**2. Microwave plasma sintering of nanoparticles**

) particles is presented.

**2.1. Sintering of nickel oxide (NiO) nanoparticles**

*2.1.1. Rapid discharge sintering vs. conventional furnace sintering*

), used as electrodes in dye sensitized solar cells (DSSCs). The effect of the plasma

) layers on the metal surface.

ide (TiO<sup>2</sup>

112 Recent Advances in Porous Ceramics

dioxide (TiO<sup>2</sup>

is discussed [42, 46].

previously [47–51]. In this study, a treatment temperature of approximately 450°C was used for a 5-min period for both the furnace and RDS treatments. This treatment time was chosen because it yielded the best photovoltaic performance for RDS treatment. Overall, a cycle time of ~120 min was required for the furnace sintering including heat up, maintenance at the max 450°C temperature and cool down. In contrast, the RDS processing required only 15 min, included all stages such as pump down, plasma treatment, and cooling.

In this Section, the properties of the furnace sintered coatings, such as morphology, crystal structure, dye adsorption, chemical composition and photovoltaic performance are compared with the NiO coatings treated for 5 min, using the RDS technique.

#### *2.1.1.1. Morphological analyses of porous NiO coatings*

A typical SEM micrograph of the morphology of RDS and furnace treated coatings is given in **Figure 3**. A higher level of interconnectivity with reduced grain growth for the RDS treated coatings when compared with the furnace sintered coatings can be observed. The increased grain growth in the furnace sintered coatings could be due to a slower cooling rate (10°C/min) used in the furnace process. In contrast, the RDS technique requires only a 5-min cooling time following the plasma treatment.

A further examination of NiO coatings using an FIB/SEM cross section micrograph (**Figure 4**) indicated that the RDS treated coating displays an improved level of porosity as well as interfacial connection between the NiO coating and FTO substrate. This could be due to the difference in the type of heating mechanisms in both sintering techniques. RDS treatment encompasses volumetric heating, which delivers more efficient heating inside the NiO coating matrix than that obtained with furnace sintering where a conductive type of heating is

**Figure 3.** SEM images of ~2.5 μm thick NiO coatings treated using (a) RDS (b) furnace sintering [52].

present. Also, furnace sintering could have resulted in inhomogeneous heating of coatings' surface to yield a heat affected zone [53]; thus, a more open structure is observed for the RDS treated coatings when compared with the furnace sintered samples.

#### *2.1.1.2. Crystal structure, crystallite size and dye adsorption evaluations of NiO coatings*

As shown in **Figure 3**, the NiO grain size obtained using the RDS process was smaller than that obtained after treatment using the furnace. A measurement of the crystallite size of NiO coatings obtained using similar sintering techniques to **Figure 3** indicated that the sizes were approximately 6.5 and 14.0 nm, respectively [42]. Thus, the RDS technique results in a finer grain size, in addition to a more uniform heating/sintering of the NiO nanoparticles.

The effect of a smaller grain size and a more open structure (**Figures 3** and **4**) obtained using the RDS technique may be observed in the UV-vis absorption spectra of the NiO coatings obtained using the two sintering techniques (**Figure 5**). The level of dye adsorption increased up to 44% for the case of RDS treated NiO coatings when compared with the furnace sintered coatings of the same thickness.

#### *2.1.1.3. Comparison of photovoltaic performance of porous NiO coatings*

Another parameter which can be used to portray the performance of the porous NiO structure is its photovoltaic performance (*I-V* characteristic) when evaluated as part of a *p*-type DSSCs. A comparison of the photovoltaic performance of NiO coatings prepared by various researchers is given in **Table 1**. From this Table, it is clear that the light-to-current conversion efficiency increase of almost 10-fold for the RDS treated NiO coatings (1–2 μm and 2.5 μm thick), when compared with the furnace sintered coatings. The conversion efficiency of the furnace treated coatings (1–2 μm thick) is comparable to the values reported in the literature [42, 46]. A notable observation is that a 2.5 μm thick NiO coatings, prepared using the RDS treatment, exhibited the best performance; it is likely due to improved dye adsorption with enhanced active sites, which results in better photochemical reaction.

**2.2. Sintered titanium dioxide (TiO2**

Grätzel [55], many researchers employ TiO<sup>2</sup>

Since the use of TiO<sup>2</sup>

the TiO<sup>2</sup>

Elsevier.

phase transformation of TiO<sup>2</sup>

**) nanoparticles**

**Figure 4.** FIB/SEM cross section images of NiO coatings obtained after 5 min sintering using (a) the furnace and (b) the RDS technique. Both coating thicknesses were approximately 2.5 μm. Reprinted from [42] with permission from

ever, as discussed earlier, it is time consuming. In this section, we will present the use of

phase(s). The effect of carbon doping on the resultant coating is also discussed. In this review,

coating of interest with respect to sintering will be that obtained by Dang et al. [56],

RDS as viable unconventional sintering technique to convert amorphous TiO<sup>2</sup>

as a photoanode in the fabrication of a DSSC in 1991 by O'Regan and

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115

coatings is generally carried out using furnace sintering, how-

for DSSC and other related applications. The

to crystalline

Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings http://dx.doi.org/10.5772/intechopen.71686 115

**Figure 4.** FIB/SEM cross section images of NiO coatings obtained after 5 min sintering using (a) the furnace and (b) the RDS technique. Both coating thicknesses were approximately 2.5 μm. Reprinted from [42] with permission from Elsevier.

#### **2.2. Sintered titanium dioxide (TiO2 ) nanoparticles**

present. Also, furnace sintering could have resulted in inhomogeneous heating of coatings' surface to yield a heat affected zone [53]; thus, a more open structure is observed for the RDS

As shown in **Figure 3**, the NiO grain size obtained using the RDS process was smaller than that obtained after treatment using the furnace. A measurement of the crystallite size of NiO coatings obtained using similar sintering techniques to **Figure 3** indicated that the sizes were approximately 6.5 and 14.0 nm, respectively [42]. Thus, the RDS technique results in a finer

The effect of a smaller grain size and a more open structure (**Figures 3** and **4**) obtained using the RDS technique may be observed in the UV-vis absorption spectra of the NiO coatings obtained using the two sintering techniques (**Figure 5**). The level of dye adsorption increased up to 44% for the case of RDS treated NiO coatings when compared with the furnace sintered

Another parameter which can be used to portray the performance of the porous NiO structure is its photovoltaic performance (*I-V* characteristic) when evaluated as part of a *p*-type DSSCs. A comparison of the photovoltaic performance of NiO coatings prepared by various researchers is given in **Table 1**. From this Table, it is clear that the light-to-current conversion efficiency increase of almost 10-fold for the RDS treated NiO coatings (1–2 μm and 2.5 μm thick), when compared with the furnace sintered coatings. The conversion efficiency of the furnace treated coatings (1–2 μm thick) is comparable to the values reported in the literature [42, 46]. A notable observation is that a 2.5 μm thick NiO coatings, prepared using the RDS treatment, exhibited the best performance; it is likely due to improved dye adsorption with

treated coatings when compared with the furnace sintered samples.

*2.1.1.3. Comparison of photovoltaic performance of porous NiO coatings*

enhanced active sites, which results in better photochemical reaction.

coatings of the same thickness.

114 Recent Advances in Porous Ceramics

*2.1.1.2. Crystal structure, crystallite size and dye adsorption evaluations of NiO coatings*

**Figure 3.** SEM images of ~2.5 μm thick NiO coatings treated using (a) RDS (b) furnace sintering [52].

grain size, in addition to a more uniform heating/sintering of the NiO nanoparticles.

Since the use of TiO<sup>2</sup> as a photoanode in the fabrication of a DSSC in 1991 by O'Regan and Grätzel [55], many researchers employ TiO<sup>2</sup> for DSSC and other related applications. The phase transformation of TiO<sup>2</sup> coatings is generally carried out using furnace sintering, however, as discussed earlier, it is time consuming. In this section, we will present the use of RDS as viable unconventional sintering technique to convert amorphous TiO<sup>2</sup> to crystalline phase(s). The effect of carbon doping on the resultant coating is also discussed. In this review, the TiO<sup>2</sup> coating of interest with respect to sintering will be that obtained by Dang et al. [56],

obtained by introducing low concentrations of carbon dioxide into the argon/oxygen plasma during the sputtering of the metal. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous

CAP microwave plasma heat treatment using a nitrogen plasma. During the plasma treatment, the substrate temperature was about 550°C. At this temperature and for treatment times as short as 1 min, 0.25 μm thick coatings converted into the anatase crystalline

tase-to-rutile crystalline phase transformation [56]. As reported earlier for the NiO layers the use of microwave plasma heat treatments facilitated a much more rapid processing compared with furnace heat treatments. It was also observed that the plasma treated TiO<sup>2</sup> coatings also exhibited higher photocurrent density. This is possibly as a result of higher level of surface roughness and consequently a higher available surface area observed for

ness when plasma treated compared to furnace treated (**Figure 6**). To investigate the phase transformation efficiency of the two treatment methods as well as the effect of carbon doping,

anatase phase peaks were observed for coatings treated between 550 and 850°C.As the temperature is increased to 875°C, peaks indicative of the rutile phase could be observed. Interestingly,

thus, it is possible that carbon doping lowered the transitional phase change temperature of

current density (IPh) measurements were obtained, as presented in **Table 2**. The highest value

In order to evaluate the effect of doping on the efficiency of the TiO<sup>2</sup>

. Further treatments of the coatings at higher temperatures resulted in ana-

Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings

coatings, C-doped coatings also exhibited a higher rough-

coatings, the rutile phase can be observed at 750°C (**Figure 7(b)**);

coatings was obtained. As shown in **Figure 7(a)**,

(a) furnace-treated and (b) MW plasma-treated at 750°C for 3 min.

coated electrodes, photo-

. This was achieved using the

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117

structure into the photoactive crystalline phase(s) of TiO<sup>2</sup>

phase of TiO<sup>2</sup>

these coatings [56].

for the C-doped TiO<sup>2</sup>

anatase to rutile.

As for the plasma treated TiO<sup>2</sup>

an XRD profile of undoped and C-doped TiO<sup>2</sup>

**Figure 6.** Surface morphology of 2.2% C-doped TiO<sup>2</sup>

Reprinted from [56] with permission from Elsevier.

**Figure 5.** UV-vis absorbance spectra dye sensitized NiO coatings of 1–2 μm thickness (RDS and furnace sintered) and ERY dye in solution. Reprinted from [42] with permission from Elsevier.


**Table 1.** Photovoltaic performance of RDS and furnace NiO coatings used as photocathode in construction of p-type DSSC (compared with the literature). AM 1.5 solar simulator (*I*: 870 W m−2), 0.5 M LiI and 0.05 M I<sup>2</sup> in propylene carbonate as an electrolyte [52]. Values in italics represent reported literature data.

for example. The following type of coatings are analyzed for their performance: furnace sintered C-doped, RDS treated undoped and C-doped RDS treated.

#### *2.2.1. Comparison of RDS and air furnace treatments of porous TiO2*

TiO<sup>2</sup> coatings were also used to fabricate DSSC electrodes. TiO<sup>2</sup> and carbon-doped TiO<sup>2</sup> were deposited as coatings onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron sputtering system [56]. The C-doped TiO<sup>2</sup> coatings were obtained by introducing low concentrations of carbon dioxide into the argon/oxygen plasma during the sputtering of the metal. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO<sup>2</sup> . This was achieved using the CAP microwave plasma heat treatment using a nitrogen plasma. During the plasma treatment, the substrate temperature was about 550°C. At this temperature and for treatment times as short as 1 min, 0.25 μm thick coatings converted into the anatase crystalline phase of TiO<sup>2</sup> . Further treatments of the coatings at higher temperatures resulted in anatase-to-rutile crystalline phase transformation [56]. As reported earlier for the NiO layers the use of microwave plasma heat treatments facilitated a much more rapid processing compared with furnace heat treatments. It was also observed that the plasma treated TiO<sup>2</sup> coatings also exhibited higher photocurrent density. This is possibly as a result of higher level of surface roughness and consequently a higher available surface area observed for these coatings [56].

As for the plasma treated TiO<sup>2</sup> coatings, C-doped coatings also exhibited a higher roughness when plasma treated compared to furnace treated (**Figure 6**). To investigate the phase transformation efficiency of the two treatment methods as well as the effect of carbon doping, an XRD profile of undoped and C-doped TiO<sup>2</sup> coatings was obtained. As shown in **Figure 7(a)**, anatase phase peaks were observed for coatings treated between 550 and 850°C.As the temperature is increased to 875°C, peaks indicative of the rutile phase could be observed. Interestingly, for the C-doped TiO<sup>2</sup> coatings, the rutile phase can be observed at 750°C (**Figure 7(b)**); thus, it is possible that carbon doping lowered the transitional phase change temperature of anatase to rutile.

In order to evaluate the effect of doping on the efficiency of the TiO<sup>2</sup> coated electrodes, photocurrent density (IPh) measurements were obtained, as presented in **Table 2**. The highest value

for example. The following type of coatings are analyzed for their performance: furnace sin-

**Table 1.** Photovoltaic performance of RDS and furnace NiO coatings used as photocathode in construction of p-type

350 420 490 560 630 700

**ERY-sensitized NiO sintered with RDS ERY-sensitized NiO sintered with furnace**

**Treatment time (min)** *VOC* **(mV)** *JSC* **(mAcm−2) FF Efficiency (***η***)**

**Erythrosin B dye in solution**

**C**

**Wavelength / nm**

**Figure 5.** UV-vis absorbance spectra dye sensitized NiO coatings of 1–2 μm thickness (RDS and furnace sintered) and

RDS treated (~2.5 μm thick) 5 120.00 1.05 36 0.0450 Furnace sintered (~2.5 μm thick) 5 84.00 0.22 25 0.0050 Furnace sintered (~2.5 μm thick) 30 35.29 0.21 26 0.0023 RDS treated (~1–2 μm thick) 5 72.14 0.53 28 0.0118 Furnace sintered (~1–2 μm thick) 5 50.30 0.24 28 0.0037 He et al. [54] (~1 μm thick) *60 83.00 0.20 27 0.0070* Nattestad et al. [48] (~1.6 μm thick) *20 120.00 0.36 26 0.0110*

were deposited as coatings onto unheated titanium and silicon wafer substrates using a

and carbon-doped TiO<sup>2</sup>

coatings were

in propylene carbonate

tered C-doped, RDS treated undoped and C-doped RDS treated.

as an electrolyte [52]. Values in italics represent reported literature data.

*2.2.1. Comparison of RDS and air furnace treatments of porous TiO2*

coatings were also used to fabricate DSSC electrodes. TiO<sup>2</sup>

DSSC (compared with the literature). AM 1.5 solar simulator (*I*: 870 W m−2), 0.5 M LiI and 0.05 M I<sup>2</sup>

DC closed-field magnetron sputtering system [56]. The C-doped TiO<sup>2</sup>

TiO<sup>2</sup>

0.0

**Awais et al. [42, 46]/***Reported data* **(NiO** 

**thickness)**

0.5

1.0

1.5

**Absorbance / a.u.**

2.0

2.5

116 Recent Advances in Porous Ceramics

**A B C**

**A**

**B**

ERY dye in solution. Reprinted from [42] with permission from Elsevier.

**Figure 6.** Surface morphology of 2.2% C-doped TiO<sup>2</sup> (a) furnace-treated and (b) MW plasma-treated at 750°C for 3 min. Reprinted from [56] with permission from Elsevier.

**Figure 7.** XRD profile of as deposited (a) undoped and (b) 2.2% C-doped TiO<sup>2</sup> upon RDS treatment. Reprinted from [56] with permission from Elsevier.


**3. Direct porous ceramics growth from metal substrates**

coatings. Reprinted from [56] with permission from Elsevier.

Compared to furnace heat treatments, microwave plasma-treated TiO<sup>2</sup>

**3.1. RDS oxidation of titanium metal substrates**

surface or alternatively as TiO<sup>2</sup>

treated TiO<sup>2</sup>

In this study, the performance of microwave plasma treatments as a technique for the oxidation of metallic surfaces is investigated. Oxides of titanium formed either on the metal

**Figure 8.** Band gap measurements for as-deposited, furnace treated, RDS treated (2.2% carbon doped) and undoped RDS

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areas ranging from medical devices (cell attachment), solar cells (light capture), air and water purification, gas sensing, wear protective coatings, etc. [57–62]. This is due to the oxide's photocatalytic, biocompatibility properties, as well as its physical and chemical stability. These properties would depend on the morphology, surface roughness and porosity of the oxides.

Section 2.2 possess higher level of surface roughness and hence photoactivity. However, the enhanced surface roughness is still only in the order of a few nanometers and thus, the photoactivity of the resulting coatings is relatively limited [63]. An alternative fabrication method

The plasma treatments were carried out as before using the CAP microwave reactor, in this case using an oxygen discharge [63, 64]. Prior to plasma oxidation, the titanium disc test

which has been investigated to address this shortcoming is presented in this section.

coatings (discussed in Section 2.2) have found applications in

coatings discussed in

**Table 2.** Photocurrent density values for different TiO<sup>2</sup> coatings.

of photocurrent density was obtained for the case of carbon doped plasma treated TiO<sup>2</sup> coatings. There was a 19% increase in the IPh value in comparison to those treated in the furnace. Possible reasons for the superior performance could be due to the enhanced porosity level of plasma treated coatings, which may have provided more active sites for charge production [56]. A further factor could be that the retention of more doped carbon in the RDS-treated TiO<sup>2</sup> coatings further reduced the band gap of RDS-treated compared to the furnace-treated TiO<sup>2</sup> coatings (See **Figure 8**).

#### **2.3. Conclusion and potential of the RDS sintering technique**

RDS-treated NiO coatings were found to exhibit a light-to-current conversion efficiency increase of almost a 10-fold, compared with that obtained for the furnace treated oxide coating. Amongst the likely reasons for the enhanced performance is the smaller grain size, along with the more open structure obtained using the RDS technique.

Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings http://dx.doi.org/10.5772/intechopen.71686 119

**Figure 8.** Band gap measurements for as-deposited, furnace treated, RDS treated (2.2% carbon doped) and undoped RDS treated TiO<sup>2</sup> coatings. Reprinted from [56] with permission from Elsevier.

## **3. Direct porous ceramics growth from metal substrates**

In this study, the performance of microwave plasma treatments as a technique for the oxidation of metallic surfaces is investigated. Oxides of titanium formed either on the metal surface or alternatively as TiO<sup>2</sup> coatings (discussed in Section 2.2) have found applications in areas ranging from medical devices (cell attachment), solar cells (light capture), air and water purification, gas sensing, wear protective coatings, etc. [57–62]. This is due to the oxide's photocatalytic, biocompatibility properties, as well as its physical and chemical stability. These properties would depend on the morphology, surface roughness and porosity of the oxides.

Compared to furnace heat treatments, microwave plasma-treated TiO<sup>2</sup> coatings discussed in Section 2.2 possess higher level of surface roughness and hence photoactivity. However, the enhanced surface roughness is still only in the order of a few nanometers and thus, the photoactivity of the resulting coatings is relatively limited [63]. An alternative fabrication method which has been investigated to address this shortcoming is presented in this section.

#### **3.1. RDS oxidation of titanium metal substrates**

of photocurrent density was obtained for the case of carbon doped plasma treated TiO<sup>2</sup>

**2.3. Conclusion and potential of the RDS sintering technique**

**Figure 7.** XRD profile of as deposited (a) undoped and (b) 2.2% C-doped TiO<sup>2</sup>

 **coatings Photocurrent density values, IPh (μA/cm2**

with the more open structure obtained using the RDS technique.

coatings (See **Figure 8**).

with permission from Elsevier.

118 Recent Advances in Porous Ceramics

As-deposited (C-doped) 108 Furnace sintered (C-doped) 181 RDS treated (un-doped) 167 RDS treated (C-doped) 216

**Table 2.** Photocurrent density values for different TiO<sup>2</sup>

**TiO2**

ings. There was a 19% increase in the IPh value in comparison to those treated in the furnace. Possible reasons for the superior performance could be due to the enhanced porosity level of plasma treated coatings, which may have provided more active sites for charge production [56]. A further factor could be that the retention of more doped carbon in the RDS-treated TiO<sup>2</sup> coatings further reduced the band gap of RDS-treated compared to the furnace-treated TiO<sup>2</sup>

coatings.

RDS-treated NiO coatings were found to exhibit a light-to-current conversion efficiency increase of almost a 10-fold, compared with that obtained for the furnace treated oxide coating. Amongst the likely reasons for the enhanced performance is the smaller grain size, along

coat-

upon RDS treatment. Reprinted from [56]

**)**

The plasma treatments were carried out as before using the CAP microwave reactor, in this case using an oxygen discharge [63, 64]. Prior to plasma oxidation, the titanium disc test substrates were polished to a mirror finish and then solvent cleaned. The oxidized substrates exhibited a white appearance, as shown in **Figure 9**; this is in contrast to the metallic appearance of the non-oxidized titanium metal.

A typical FIB/metallography SEM cross-section image of a RDS treated TiO<sup>2</sup> structure obtained using a focused ion beam (FIB) and metallographic technique for comparison is shown in **Figure 10**. It can be observed that the two sample preparation methods (FIB and metallography) did not alter the porous structure of the ceramics.

#### **3.2. Influence of substrate temperature on the pore structure morphology in a RDS grown oxide-layer**

The porosity of the oxide-layer structure was found to generally increase with increase in thickness and treatment temperature (**Figure 11**). Examination of the porous oxide structures indicates that oxide-layers grown at temperature below 880°C exhibits a relatively porous structure, in the upper oxide layer compared to that closer to the metal substrate. Oxides fabricated above 880°C exhibited a further increase in porosity levels throughout the oxide layer. It is well known that α to β phase transformation of titanium occurs at approximately 882°C; this may have influenced porosity distribution observed in the oxide-layers grown on titanium substrates above 880°C [64].

#### **3.3. Comparison of the porosity of sintered TiO2 , air furnace and RDS grown oxide-layer**

**Figure 12** shows the oxide layer morphology of a RDS and air furnace grown ceramic structures. The structure in **Figure 12(a)** was obtained after 5-hours furnace treatment in air and its thickness was found to be 4.17 μm. In contrast, after treatment in an oxygen microwave plasma for 10 min, the thickness obtained was 6.96 μm (**Figure 12(b)**). It can be observed that the oxide-layer obtained using the microwave plasma oxidation exhibited a relatively

rough morphology, with large grains. In contrast, the slower growing oxide formed using the furnace oxidation exhibited a much denser morphology with little or no porosity observed. A possible reason for the increased porosity obtained for the RDS treatments is a result of a preferential grain growth in specific directions due to a van der Drift type of competition with

**Figure 11.** Oxide-layers (demonstrated using the arrows), which were grown on titanium substrates at temperature: (a) 855°C, (b) 880°C and (c) 910°C showing porosity distribution. Note the increased porosity obtained at the higher

**Figure 10.** SEM images of oxide cross-section prepared using the FIB (a) and metallographic technique (b). The images were obtained using an FEI Quanta 3D FEG DualBeam system. The average oxide-layer thickness for both techniques is

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9.89 μm. (Note a 52° tilt angle is used for the FIB image.) Reprinted from [64] with permission from Elsevier.

as part of this study as shown in **Figure 13**. **Figure 13(a)** was obtained by oxidation of metal oxide nanoparticles and **Figure 13(b)** by the oxidation of the titanium metal. Both were obtained using the microwave plasma treatments. It is clear from **Figure 13** that the sintering

tion, however, involves two steps (spray deposition and sintering), compared with the single microwave plasma oxidation step. Both treatments, however, demonstrate the flexibility of

nanoparticles yields a much more homogeneous oxide pore structure. Its fabrica-

oxide layers obtained

To conclude it is interesting to compare the morphology of two TiO<sup>2</sup>

oxidation temperature. Reprinted from [64] with permission from Elsevier.

increasing thickness of oxide layer [64].

the microwave plasma treatments.

of the TiO<sup>2</sup>

**Figure 9.** Photograph of oxide-layer formed on the 25-mm diameter titanium disc using microwave plasma and air furnace. Reprinted from [64] with permission from Elsevier.

Microwave Plasmas as a Processing Tool for Tailoring the Surface Properties of Ceramic Coatings http://dx.doi.org/10.5772/intechopen.71686 121

substrates were polished to a mirror finish and then solvent cleaned. The oxidized substrates exhibited a white appearance, as shown in **Figure 9**; this is in contrast to the metallic appear-

obtained using a focused ion beam (FIB) and metallographic technique for comparison is shown in **Figure 10**. It can be observed that the two sample preparation methods (FIB and

**3.2. Influence of substrate temperature on the pore structure morphology in a RDS grown** 

The porosity of the oxide-layer structure was found to generally increase with increase in thickness and treatment temperature (**Figure 11**). Examination of the porous oxide structures indicates that oxide-layers grown at temperature below 880°C exhibits a relatively porous structure, in the upper oxide layer compared to that closer to the metal substrate. Oxides fabricated above 880°C exhibited a further increase in porosity levels throughout the oxide layer. It is well known that α to β phase transformation of titanium occurs at approximately 882°C; this may have influenced porosity distribution observed in the oxide-layers grown on

**Figure 12** shows the oxide layer morphology of a RDS and air furnace grown ceramic structures. The structure in **Figure 12(a)** was obtained after 5-hours furnace treatment in air and its thickness was found to be 4.17 μm. In contrast, after treatment in an oxygen microwave plasma for 10 min, the thickness obtained was 6.96 μm (**Figure 12(b)**). It can be observed that the oxide-layer obtained using the microwave plasma oxidation exhibited a relatively

**Figure 9.** Photograph of oxide-layer formed on the 25-mm diameter titanium disc using microwave plasma and air

structure

**, air furnace and RDS grown oxide-layer**

A typical FIB/metallography SEM cross-section image of a RDS treated TiO<sup>2</sup>

metallography) did not alter the porous structure of the ceramics.

ance of the non-oxidized titanium metal.

120 Recent Advances in Porous Ceramics

titanium substrates above 880°C [64].

**3.3. Comparison of the porosity of sintered TiO2**

furnace. Reprinted from [64] with permission from Elsevier.

**oxide-layer**

**Figure 10.** SEM images of oxide cross-section prepared using the FIB (a) and metallographic technique (b). The images were obtained using an FEI Quanta 3D FEG DualBeam system. The average oxide-layer thickness for both techniques is 9.89 μm. (Note a 52° tilt angle is used for the FIB image.) Reprinted from [64] with permission from Elsevier.

**Figure 11.** Oxide-layers (demonstrated using the arrows), which were grown on titanium substrates at temperature: (a) 855°C, (b) 880°C and (c) 910°C showing porosity distribution. Note the increased porosity obtained at the higher oxidation temperature. Reprinted from [64] with permission from Elsevier.

rough morphology, with large grains. In contrast, the slower growing oxide formed using the furnace oxidation exhibited a much denser morphology with little or no porosity observed. A possible reason for the increased porosity obtained for the RDS treatments is a result of a preferential grain growth in specific directions due to a van der Drift type of competition with increasing thickness of oxide layer [64].

To conclude it is interesting to compare the morphology of two TiO<sup>2</sup> oxide layers obtained as part of this study as shown in **Figure 13**. **Figure 13(a)** was obtained by oxidation of metal oxide nanoparticles and **Figure 13(b)** by the oxidation of the titanium metal. Both were obtained using the microwave plasma treatments. It is clear from **Figure 13** that the sintering of the TiO<sup>2</sup> nanoparticles yields a much more homogeneous oxide pore structure. Its fabrication, however, involves two steps (spray deposition and sintering), compared with the single microwave plasma oxidation step. Both treatments, however, demonstrate the flexibility of the microwave plasma treatments.

to yield improved results in terms of shorter treatment time, lower energy requirement and enhanced performance. The latter was demonstrated in the case of metal oxide layers (NiO and

1 School of Mechanical and Materials Engineering, University College Dublin, Dublin 4,

[1] Clark DE, Folz DC, West JK. Processing materials with microwave energy. Materials

[2] Das S, Mukhopadhyay AK, Datta S, Basu D. Prospects of microwave processing: An

[3] Sutton WH. Microwave processing of ceramics-an overview. In: MRS Proceedings. Vol.

[4] Agrawal D. Microwave sintering of metal powders. In: Chang I, Zhao Y, editors. Advances in Powder Metallurgy. Woodhead Publishing Ltd; Sawston, UK, 2013.

[5] Thostenson ET, Chou TW. Microwave processing: Fundamentals and applications. Composites Part A: Applied Science and Manufacturing. 1999;**30**(9):1055-1071

[6] Roy R, Agrawal D, Cheng J, Gedevanishvili S. Full sintering of powdered-metal bodies

[7] Sethi G, Upadhyaya A, Agrawal D. Microwave and conventional sintering of premixed

[8] Takayama S, Saiton Y, Sato M, Nagasaka T, Muroga T, Ninomiya Y. Microwave sintering for metal powders in the air by non-thermal effect. In: Proceedings of the 9th Conference

[9] Gupta M, Wong WLE. Enhancing overall mechanical performance of metallic materials using two-directional microwave assisted rapid sintering. Scripta Materialia. 2005;

and prealloyed Cu-12Sn bronze. Science of Sintering. 2003;**35**(2):49-65

on Microwave and High Frequency Heating; 2003. pp. 369-372

2 Department of Industrial Engineering, Taibah University, Medina, Saudi Arabia

and Denis P. Dowling1

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), when evaluated for use in solar energy applications.

\*, Muhammad Awais2

Science and Engineering: A. 2000;**287**(2):153-158

overview. Bulletin of Materials Science. 2009;**32**(1):1-13

in a microwave field. Nature. 1999;**399**(6737):668-670

269. Cambridge University Press; Cambridge, UK, 1992. p. 3

\*Address all correspondence to: emmanuel.ekoi@ucdconnect.ie

TiO<sup>2</sup>

Ireland

**References**

p. 361-379

**52**(6):479-483

**Author details**

Emmanuel J. Ekoi1

**Figure 12.** Typical SEM images of oxide-layers grown using a RDS (a); and air furnace (b), demonstrating differences in the grain structure obtained (samples both prepared at a treatment temperature of 790°C). Reprinted from [64] with permission from Elsevier.

**Figure 13.** FIB cross-sections of an 8.5 μm thick TiO<sup>2</sup> coating (a) fabricated by sintering TiO<sup>2</sup> nanoparticles. The FIB cross section (b) shows a 10.45 μm TiO<sup>2</sup> oxide-layer obtained by oxidizing the Cp titanium metal (reprinted from [64] with permission from Elsevier).
