**3. Physical properties of alumina-graphene hybrids for technological and applied applications**

TGA (shown in **Figure 3**) of γ-Al2O3-rGO powder samples show that different calcination times has led to different concentrations of rGO in the hybrids. The TGA curves of all hybrids show a stable weight loss between 400 and 600°C, as a result of the removal of all carbon-related materials, and other impurities (if any) after heating these hybrids to 800°C in an air atmosphere. For the samples with 3-, 2- and 1-h calcination time, the 7.705, 12.830 and 16.707 wt.% loss were calculated. For 1-h sample, the unique weight loss is observed.

SEM image in (**Figure 4a**) for bare Al2O3 shows particles like morphology. The size of particles has ranged from 500 nm to few micrometers. TEM image in (**Figure 4b**) shows elongated nanocrystals or nanorods of bare Al2O3. Sample before calcination but after autoclave heating has been referred as Al(O)x/rGO. SEM image of Al(O)x/rGO after heating in an autoclave at a temperature of 453 K for 6 h but before calcination is shown in the **Figure 4c**. Even after calcination at 723 K for 2 h, the SEM image in **Figure 4d** shows the same particle like morphology but size of particles has ranged from 1 micrometers to few micrometers. TEM image of γ-Al2O3-rGO hybrids after calcination at 723 K for 2 h is shown in the **Figure 4e**. It shows elongated and fine nanorods of γ-Al2O3 with rGO layer in hybrids. The TEM image in **Figure 4e** indicates the presence of a very thin rGO layer, which acts as a continuum matrix in these hybrids.

The presence of rGO can also be confirmed by closely observing **Figure 4e**. In this figure, low-contrast features are actually edges or small portions of the graphene sheet (**Figure 4e**) on which γ-Al2O3 is uniformly distributed in dense concentrations. Further, the selected area electron diffraction pattern presented in **Figure 4f** shows the inter-planar spacing's of D = 0.175 nm and D = 0.151 nm, corresponding to the (200) and (111) planes of γ-Al2O3. The fabrication of the γ-Al2O3 phase was

**Figure 3.** *TGA curves of γ-Al2O3-rGO hybrids using calcination time of 1, 2 and 3 h in air atmosphere up to 800°C.*

*Silicon Materials*

added drop by drop to the GO suspension above. Centrifugation was used to separate the products which were then washed out several times with cyclohexane. The solid samples thus obtained are denoted as (Ceramics Oxide)x/GO. (Ceramic Oxide) x/GO was dispersed again in 50 ml cyclohexane and then transferred to a 100 ml Teflon-lined stainless-steel autoclave for hydrothermal reaction. After the reaction was carried out, samples denoted as (Ceramic Oxide)x/rGO. (Ceramic Oxide)x/ rGO was then calcinated at a temperature above 700 K for a specific interval of time to form Ceramics Oxide/rGO hybrids. Graphene-ceramics hybrid powder containing different wt.% of rGO were obtained using the same method. Hot pressing of ceramics-graphene hybrid powder was performed in a vacuum furnace (model number OTF-1200X-VHP4). The flowchart fabrication scheme of gamma aluminarGO hybrid with detailed experimental conditions is represented in **Figure 1**.

**162**

**Figure 2.**

**Figure 1.**

*Flow chart fabrication scheme for SiO2-rGO hybrids.*

*Flow chart fabrication scheme for γ-Al2O3-rGO hybrids.*

confirmed from the XRD results as shown in **Figures 5** and **6**, respectively. The inset of **Figure 6** shows the XRD of a sample without GO (γ-Al2O3). In the XRD spectra of all three samples with calcination times of 1, 2, and 3 h, the presence of characteristic peaks of γ-Al2O3 is evident and (matched with JCPDS card no. 10-0425). Further, a relatively broad nature of sharp peaks is observed in the XRD of γ-Al2O3 and rGO hybrids (after calcination, **Figure 5**) and in the inset of **Figure 6** (for pure γ-Al2O3). This suggests a nanocrystalline structure of γ-Al2O3 with nanorod morphology, which is also quite evident from the TEM images (**Figure 4e**).

#### **Figure 4.**

*(a) SEM and (b) TEM images of pure γ-Al2O3. SEM images of Al(O)x/GO (c) before calcination at autoclave heating of 453 K for 6 h and (d) after calcination at 723 K for 2 h. (e) TEM image of γ-Al2O3-rGO after calcination at 723 K for 2 h, and (f) SAED pattern of γ-Al2O3-rGO hybrid at 723 K for 2 h.*

#### **Figure 5.**

*XRD of γ-Al2O3-rGO (1-h calcination time), γ-Al2O3-rGO (2-h calcination time) and γ-Al2O3-rGO (3-h calcination time).*

**165**

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

In all cases after calcination for 1, 2 and 3 h in **Figure 5**, the characteristic peaks

Raman spectroscopy is conducted on these hybrids to confirm the presence of carbon, shown in **Figure 7**. The Raman spectra of the sample indicate that rGO is present in hybrids with 1-, 2- and 3-h calcination time. For calcinated samples with different times (1, 2, and 3 h), the Raman intensity decreases with calcination time, consistent with TGA results. The G-band value is different in the Raman spectrum of all γ-Al2O3 and rGO hybrids as compared to pristine graphene. This reveals the presence of prominent electronic interactions between γ-Al2O3 and rGO in hybrids. The nanohybrids of γ-Al2O3-rGO with 3, 2, and 1-h calcination time have BET

of GO that usually appear at 10.28 are also invisible in these hybrids. The total changes in the peaks of the XRD pattern before and after calcination indicate the

*XRD of γ-Al2O3-rGO before calcination and inset is XRD of pure γ-Al2O3, fabricated at 723 K.*

•g<sup>−</sup><sup>1</sup>

BET surface-area values. Presence of rGO in hybrid can increase the BET surface area, pore volume, and thermal conductivity. The higher surface areas, pore volume, thermal and electrical conductivity are significant factors from an applied application point of view. Further as the calcination temperature increased, the crystallinity of γ-Al2O3 and rGO hybrids was considerably enhanced as shown in the XRD of γ-Al2O3 and rGO hybrids which are taken from 500 to 800 K as in **Figure 8**. For the analysis, calcination time is kept constant (1 h) and further calcination temperature was set as 500, 600, 650, 700, 750, and 800 K, respectively. Actually, most of the samples do not show any characteristics peaks for the rGO in XRD (**Figure 8**). Now, the crystallinity of γ-alumina is a major concern. Further, it is found that sharp, broad and prominent peaks are obtained for γ-alumina at the higher calcination temperature. But the peaks were weaker or not prominent, when there was a lower calcination temperature such as 500, 600, and 650, respectively.

, respectively. For bare γ-Al2O3, the BET

. Clearly, the presence of rGO has led to high

, respectively. For bare

. The bulk densities of γ-Al2O3-rGO with 3-, 2-, and 1-h

effects of hydrothermal and calcination treatments.

surface areas of 361, 408, and 379 m2

γ-Al2O3, the bulk density is 2.75 g cm<sup>−</sup><sup>3</sup>

•g<sup>−</sup><sup>1</sup>

calcination time have values of 1.61, 1.37, and 0.92 g cm<sup>−</sup><sup>3</sup>

surface area is 280 m2

**Figure 6.**

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

**Figure 6.** *XRD of γ-Al2O3-rGO before calcination and inset is XRD of pure γ-Al2O3, fabricated at 723 K.*

In all cases after calcination for 1, 2 and 3 h in **Figure 5**, the characteristic peaks of GO that usually appear at 10.28 are also invisible in these hybrids. The total changes in the peaks of the XRD pattern before and after calcination indicate the effects of hydrothermal and calcination treatments.

Raman spectroscopy is conducted on these hybrids to confirm the presence of carbon, shown in **Figure 7**. The Raman spectra of the sample indicate that rGO is present in hybrids with 1-, 2- and 3-h calcination time. For calcinated samples with different times (1, 2, and 3 h), the Raman intensity decreases with calcination time, consistent with TGA results. The G-band value is different in the Raman spectrum of all γ-Al2O3 and rGO hybrids as compared to pristine graphene. This reveals the presence of prominent electronic interactions between γ-Al2O3 and rGO in hybrids.

The nanohybrids of γ-Al2O3-rGO with 3, 2, and 1-h calcination time have BET surface areas of 361, 408, and 379 m2 •g<sup>−</sup><sup>1</sup> , respectively. For bare γ-Al2O3, the BET surface area is 280 m2 •g<sup>−</sup><sup>1</sup> . The bulk densities of γ-Al2O3-rGO with 3-, 2-, and 1-h calcination time have values of 1.61, 1.37, and 0.92 g cm<sup>−</sup><sup>3</sup> , respectively. For bare γ-Al2O3, the bulk density is 2.75 g cm<sup>−</sup><sup>3</sup> . Clearly, the presence of rGO has led to high BET surface-area values. Presence of rGO in hybrid can increase the BET surface area, pore volume, and thermal conductivity. The higher surface areas, pore volume, thermal and electrical conductivity are significant factors from an applied application point of view. Further as the calcination temperature increased, the crystallinity of γ-Al2O3 and rGO hybrids was considerably enhanced as shown in the XRD of γ-Al2O3 and rGO hybrids which are taken from 500 to 800 K as in **Figure 8**. For the analysis, calcination time is kept constant (1 h) and further calcination temperature was set as 500, 600, 650, 700, 750, and 800 K, respectively. Actually, most of the samples do not show any characteristics peaks for the rGO in XRD (**Figure 8**).

Now, the crystallinity of γ-alumina is a major concern. Further, it is found that sharp, broad and prominent peaks are obtained for γ-alumina at the higher calcination temperature. But the peaks were weaker or not prominent, when there was a lower calcination temperature such as 500, 600, and 650, respectively.

*Silicon Materials*

confirmed from the XRD results as shown in **Figures 5** and **6**, respectively. The inset of **Figure 6** shows the XRD of a sample without GO (γ-Al2O3). In the XRD spectra of all three samples with calcination times of 1, 2, and 3 h, the presence of characteristic peaks of γ-Al2O3 is evident and (matched with JCPDS card no. 10-0425). Further, a relatively broad nature of sharp peaks is observed in the XRD of γ-Al2O3 and rGO hybrids (after calcination, **Figure 5**) and in the inset of **Figure 6** (for pure γ-Al2O3). This suggests a nanocrystalline structure of γ-Al2O3 with nanorod

morphology, which is also quite evident from the TEM images (**Figure 4e**).

*(a) SEM and (b) TEM images of pure γ-Al2O3. SEM images of Al(O)x/GO (c) before calcination at autoclave heating of 453 K for 6 h and (d) after calcination at 723 K for 2 h. (e) TEM image of γ-Al2O3-rGO after* 

*XRD of γ-Al2O3-rGO (1-h calcination time), γ-Al2O3-rGO (2-h calcination time) and γ-Al2O3-rGO (3-h* 

*calcination at 723 K for 2 h, and (f) SAED pattern of γ-Al2O3-rGO hybrid at 723 K for 2 h.*

**164**

**Figure 5.**

*calcination time).*

**Figure 4.**

#### **Figure 7.**

*Raman spectra of γ-Al2O3-rGO (1-h calcination time), γ-Al2O3-rGO (2-h calcination time) and γ-Al2O3-rGO (3-h calcination time).*

#### **Figure 8.**

*XRD of γ-Al2O3-rGO hybrids taken from 500 to 800 K.*

Hot-pressed γ-Al2O3 and rGO nanohybrid samples were fabricated at a temperature of 900°C. Hot pressing can affect the quality of graphene. Preserving the quality of graphene as much as possible is a major factor in the enhanced properties of graphene hybrids [19–21]. The SEM morphology of all samples after hot pressing is shown in the **Figure 9**.

The Raman and XRD data for γ-Al2O3-rGO calcinated at 2 h, before and after hot pressing, are shown in **Figures 10** and **11**, respectively. Before hot pressing,

**167**

at 1350.89 cm<sup>−</sup><sup>1</sup>

**Figure 10.**

**Figure 9.**

the D band for the sample is found at 1345.21 cm<sup>−</sup><sup>1</sup>

*Raman of γ-Al2O3-rGO (2-h calcination time) before and after hot-pressing.*

pressing, the G band is found at 1592.33 cm<sup>−</sup><sup>1</sup>

. Before hot pressing, the G band is found at 1588.46 cm<sup>−</sup><sup>1</sup>

*SEM images of hot pressed samples (a) γ-Al2O3-rGO (1 h calcination time), (b) γ-Al2O3-rGO (2-h calcination* 

*time), (c) γ-Al2O3-rGO (3-h calcination time), and (d) pure γ-Al2O3 (1-h calcination time).*

the electronic interaction between γ-Al2O3 and rGO during hot press processing.

. After hot pressing, it is found

. This shift in D and G bands is due to

. After hot

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

#### **Figure 9.**

*Silicon Materials*

**Figure 7.**

*(3-h calcination time).*

**166**

**Figure 8.**

shown in the **Figure 9**.

*XRD of γ-Al2O3-rGO hybrids taken from 500 to 800 K.*

Hot-pressed γ-Al2O3 and rGO nanohybrid samples were fabricated at a temperature of 900°C. Hot pressing can affect the quality of graphene. Preserving the quality of graphene as much as possible is a major factor in the enhanced properties of graphene hybrids [19–21]. The SEM morphology of all samples after hot pressing is

*Raman spectra of γ-Al2O3-rGO (1-h calcination time), γ-Al2O3-rGO (2-h calcination time) and γ-Al2O3-rGO* 

The Raman and XRD data for γ-Al2O3-rGO calcinated at 2 h, before and after hot pressing, are shown in **Figures 10** and **11**, respectively. Before hot pressing,

*SEM images of hot pressed samples (a) γ-Al2O3-rGO (1 h calcination time), (b) γ-Al2O3-rGO (2-h calcination time), (c) γ-Al2O3-rGO (3-h calcination time), and (d) pure γ-Al2O3 (1-h calcination time).*

#### **Figure 10.**

*Raman of γ-Al2O3-rGO (2-h calcination time) before and after hot-pressing.*

the D band for the sample is found at 1345.21 cm<sup>−</sup><sup>1</sup> . After hot pressing, it is found at 1350.89 cm<sup>−</sup><sup>1</sup> . Before hot pressing, the G band is found at 1588.46 cm<sup>−</sup><sup>1</sup> . After hot pressing, the G band is found at 1592.33 cm<sup>−</sup><sup>1</sup> . This shift in D and G bands is due to the electronic interaction between γ-Al2O3 and rGO during hot press processing.

There is not much effect on the integrity of alumina nanorods during hot pressing as this is also confirmed by XRD (**Figure 11**).

The three hot-pressed samples of γ-Al2O3-rGO hybrids with calcination times from 1, 2, and 3 h and pure γ-Al2O3 (calcinated at 1 h) were studied for properties studies. Electrical conductivity as a function of the concentration of rGO in the hybrid is shown in the **Figure 12**.

It is also found that the electrical conductivity increases with more rGO content. The electrical conductivities of γ-Al2O3-rGO calcinated at 1, 2, and 3 h were 8.2 × 101 , 7.8 × 101 , and 6.7 × 101 S•m<sup>−</sup><sup>1</sup> , respectively. There is conductivity (5.1 × 10<sup>−</sup>10 S•m−<sup>1</sup> ) found in bare γ-Al2O3 samples, which confirms that the bare alumina is highly nonconductive. Previous reports show that little carbon (2%) in alumina-carbon hybrids can enhance conductivity up to great level (from 10<sup>−</sup>12 S•m−<sup>1</sup> to 10<sup>−</sup><sup>1</sup> S•m<sup>−</sup><sup>1</sup> ). The improvement of the electrical properties is due to heat treatment, and it has been attributed to mechanisms such as restoration of sp2 C▬C bonds and cross-linking between reduced GO sheets during the thermal annealing process [28]. These

**Figure 11.** *XRD of γ-Al2O3-rGO (2-h calcination time) before and after hot pressing.*

**169**

**Figure 13.**

*2- and 3-h calcination time with error bar.*

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

samples showed a significant increase and improvement in thermal conductivity. This is mainly because of the excess surface electrons and the layered structure of rGO. Thin rGO layers can independently have higher conductivities. The thermal

hydrolysis and solvothermal method. The thermal conductivity of γ-Al2O3-rGO with 1-, 2- and 3-h calcination time and bare γ-Al2O3 (1-h calcination time) as a function of varying temperatures is shown in **Figure 13**. At a room temperature of 25°C, the thermal conductivities of pure γ-Al2O3 and γ-Al2O3-rGO (3, 2 and 1-h calcination

perature was increased, the thermal conductivity gradually increased in all hybrids

In the case of ceramic materials, porosity is one of the main reasons for decreases in the overall thermal conductivity. The dielectric properties of γ-Al2O3-rGO hybrids and bare γ-Al2O3 were measured using an LCR meter, as shown in **Figure 14**. For γ-Al2O3, its dielectric constant is found to be around 9.8, which is closer to that of

For the 3-h calcinated hybrid, the dielectric constant is significantly increased and is multiplied by a factor of 12, which indicates the presence of a first percolation threshold. However, when the rGO content is enhanced in the hybrids by decreasing calcination time to 2 h, the dielectric constant further decreases and approaches the value of pure γ-Al2O3. This is attributed to an anomalous trend that produces drastic changes that are usually suffered by most ceramic materials and matrix microstructures. By further decreasing the calcination temperature of hybrids to 1 h, the dielectric constant increases by four orders of magnitude, which indicates the presence of a second percolation threshold that is achieved through this higher value of dielectric constant. Similarly, the dielectric loss indicates very similar behavior in the real part of the dielectric constant as shown inset of **Figure 14**. Dielectric loss of Al2O3-rGO hybrid with 1-h calcination time is much increased, as more rGO in hybrid and this more rGO can make conductive layers' network of rGO in between alumina nanorods; this would cause significant leakage current, and thus result in a high dielectric loss. The existence of a double percolation threshold in such γ-Al2O3 and rGO hybrids can be significant for technological and applied applications because it can be used to enhance the dielectric properties in γ-Al2O3 and rGO

*Thermal conductivity as function of temperature (black, red and green curves are for γ-Al2O3-rGO with 1-,* 

K<sup>−</sup><sup>1</sup>

K<sup>−</sup><sup>1</sup>

at 75°C, which prepared by

, respectively. As the tem-

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

of γ-Al2O3-rGO and bare γ-Al2O3.

pure alumina.

conductivity of pure alumina was reported 0.5 Wm<sup>−</sup><sup>1</sup>

time) were found to be 0.81, 1.4, 2.37, and 2.53 Wm<sup>−</sup><sup>1</sup>

**Figure 12.** *Electrical conductivity vs. % rGO with error bar.*

### *Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

samples showed a significant increase and improvement in thermal conductivity. This is mainly because of the excess surface electrons and the layered structure of rGO. Thin rGO layers can independently have higher conductivities. The thermal conductivity of pure alumina was reported 0.5 Wm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> at 75°C, which prepared by hydrolysis and solvothermal method. The thermal conductivity of γ-Al2O3-rGO with 1-, 2- and 3-h calcination time and bare γ-Al2O3 (1-h calcination time) as a function of varying temperatures is shown in **Figure 13**. At a room temperature of 25°C, the thermal conductivities of pure γ-Al2O3 and γ-Al2O3-rGO (3, 2 and 1-h calcination time) were found to be 0.81, 1.4, 2.37, and 2.53 Wm<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> , respectively. As the temperature was increased, the thermal conductivity gradually increased in all hybrids of γ-Al2O3-rGO and bare γ-Al2O3.

In the case of ceramic materials, porosity is one of the main reasons for decreases in the overall thermal conductivity. The dielectric properties of γ-Al2O3-rGO hybrids and bare γ-Al2O3 were measured using an LCR meter, as shown in **Figure 14**. For γ-Al2O3, its dielectric constant is found to be around 9.8, which is closer to that of pure alumina.

For the 3-h calcinated hybrid, the dielectric constant is significantly increased and is multiplied by a factor of 12, which indicates the presence of a first percolation threshold. However, when the rGO content is enhanced in the hybrids by decreasing calcination time to 2 h, the dielectric constant further decreases and approaches the value of pure γ-Al2O3. This is attributed to an anomalous trend that produces drastic changes that are usually suffered by most ceramic materials and matrix microstructures. By further decreasing the calcination temperature of hybrids to 1 h, the dielectric constant increases by four orders of magnitude, which indicates the presence of a second percolation threshold that is achieved through this higher value of dielectric constant. Similarly, the dielectric loss indicates very similar behavior in the real part of the dielectric constant as shown inset of **Figure 14**. Dielectric loss of Al2O3-rGO hybrid with 1-h calcination time is much increased, as more rGO in hybrid and this more rGO can make conductive layers' network of rGO in between alumina nanorods; this would cause significant leakage current, and thus result in a high dielectric loss. The existence of a double percolation threshold in such γ-Al2O3 and rGO hybrids can be significant for technological and applied applications because it can be used to enhance the dielectric properties in γ-Al2O3 and rGO

#### **Figure 13.**

*Thermal conductivity as function of temperature (black, red and green curves are for γ-Al2O3-rGO with 1-, 2- and 3-h calcination time with error bar.*

*Silicon Materials*

7.8 × 101

as this is also confirmed by XRD (**Figure 11**).

S•m<sup>−</sup><sup>1</sup>

can enhance conductivity up to great level (from 10<sup>−</sup>12 S•m−<sup>1</sup>

*XRD of γ-Al2O3-rGO (2-h calcination time) before and after hot pressing.*

hybrid is shown in the **Figure 12**.

, and 6.7 × 101

There is not much effect on the integrity of alumina nanorods during hot pressing

The three hot-pressed samples of γ-Al2O3-rGO hybrids with calcination times from 1, 2, and 3 h and pure γ-Al2O3 (calcinated at 1 h) were studied for properties studies. Electrical conductivity as a function of the concentration of rGO in the

It is also found that the electrical conductivity increases with more rGO content. The electrical conductivities of γ-Al2O3-rGO calcinated at 1, 2, and 3 h were 8.2 × 101

found in bare γ-Al2O3 samples, which confirms that the bare alumina is highly nonconductive. Previous reports show that little carbon (2%) in alumina-carbon hybrids

improvement of the electrical properties is due to heat treatment, and it has been attributed to mechanisms such as restoration of sp2 C▬C bonds and cross-linking between reduced GO sheets during the thermal annealing process [28]. These

, respectively. There is conductivity (5.1 × 10<sup>−</sup>10 S•m−<sup>1</sup>

to 10<sup>−</sup><sup>1</sup>

S•m<sup>−</sup><sup>1</sup>

). The

,

)

**168**

**Figure 12.**

**Figure 11.**

*Electrical conductivity vs. % rGO with error bar.*

**Figure 14.** *Dielectric properties vs. % rGO in γ-Al2O3-rGO hybrid with error bar.*

hybrids with the addition of a small rGO % in the hybrid. From compressive and tensile stress-strain analysis, it is evident that with an increase of rGO content in the hybrid the mechanical compressive and tensile strength is increased as compared to pure alumina. This further caused more strength in alumina hybrids, i.e., higher compressive, tensile strength and higher compressive young modulus values for these hybrids (**Figures 15** and **16**). The enhanced mechanical properties of γ-Al2O3 and rGO hybrids can be attributed to covalent interaction of rGO with γ-Al2O3 and to efficient load transfers between rGO and nanorods of γ-Al2O3. Further, this is closely bound with the elongated and fine γ-Al2O3 nanorods and atomic-level rGO layers with a covalent interaction with γ-Al2O3. Young modulus of γ-Al2O3-rGO with 1-, 2- and 3-h calcination time and γ-Al2O3 with 1-h calcination time are calculated as 3.7, 3.2, 2.65 and 1.80 GPa. In this case, lower tensile and compressive strength in alumina can be due to the availability of powder instead of single crystals of alumina. Increase in calcination temperature has reduced wt.% of rGO in a hybrid. This is the reason of having more strength in hybrids with lower calcination

**171**

**Figure 16.**

**applied applications**

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications*

temperature. The maximum value of Young's modulus (3.7 GPa) is determined in

Thus, elongated dimensions of nanorods are a major cause of higher mechanical strength in these hybrids. In γ-Al2O3-rGO monoliths, higher calcination temperature enhances length, diameter and aspect ratios of γ-Al2O3 nanorods. Presence of more rGO and higher aspect ratio elongated alumina rods determines the interface interaction between rGO platelets and alumina. A 90% increase in tensile strength and 75% in compressive strength occurs when the content of rGO is increased from 0 to 7.705 wt.% in the hybrid calcination of a hybrid at 3-h processing time. With the increase of rGO, alumina-

rGO hybrids have shown higher values for young modulus. The hybrids with 1-h calcination time show good enhancement in its electrical conductivity (8.2 × 101

due to the availability of more surface electrons of rGO. This is best-reported values for conductivity. After hot press process, there is a wide increase in electrical conductivity values when there is a decrease of calcination temperature from 3 to 1-h processing time in these hybrids. Further, the thermal conductivity of γ-Al2O3-rGO is enhanced by more than 80% compared to that of bare γ-Al2O3 when there is an increase in rGO content up to 7.705 wt.% in γ-Al2O3 and rGO hybrids. There is a 77% increase in thermal conductivity using this solvothermal method in these γ-Al2O3 and rGO hybrids. Physical properties such as the BET surface area and bulk density are also improved. Elongated dimensions of nanorods are a major cause of higher mechanical strength in these hybrids. Dielectric constant increases by four orders of magnitude through second percolation threshold with the addition of small rGO in a hybrid. Enhancement in physical properties can be due to well aligned, elongated and fine nanorods morphology of alumina in hybrids, calcination, and hot press processing further played an important role by sustaining quality rGO in hybrids. These nanohybrids of alumina monoliths and rGO can be further applied as catalysts, electrolytes, and as electrochemically active materials because of

 S m<sup>−</sup><sup>1</sup> )

1-h calcinated alumina-rGO hybrid, as also shown in **Figure 16**.

*Compressive Young's modulus as % rGO in alumina-rGO hybrids.*

their nanometer dimensions and enhanced physical properties.

**4. Physical properties of silica-graphene hybrids for technological** 

With rGO in the SiO2-rGO hybrids, the hybrids powder shows a prominent change in the color after calcination. TGA of SiO2-rGO powder samples shows that

*DOI: http://dx.doi.org/10.5772/intechopen.85575*

**Figure 15.** *Compressive and tensile strength as a function of rGO for hybrid γ-Al2O3-rGO with error bars.*

*Ceramics (Si- and Al-Based Oxides)-Graphene Hybrids and Advanced Applications DOI: http://dx.doi.org/10.5772/intechopen.85575*

#### **Figure 16.** *Compressive Young's modulus as % rGO in alumina-rGO hybrids.*

temperature. The maximum value of Young's modulus (3.7 GPa) is determined in 1-h calcinated alumina-rGO hybrid, as also shown in **Figure 16**.

Thus, elongated dimensions of nanorods are a major cause of higher mechanical strength in these hybrids. In γ-Al2O3-rGO monoliths, higher calcination temperature enhances length, diameter and aspect ratios of γ-Al2O3 nanorods. Presence of more rGO and higher aspect ratio elongated alumina rods determines the interface interaction between rGO platelets and alumina. A 90% increase in tensile strength and 75% in compressive strength occurs when the content of rGO is increased from 0 to 7.705 wt.% in the hybrid calcination of a hybrid at 3-h processing time. With the increase of rGO, aluminarGO hybrids have shown higher values for young modulus. The hybrids with 1-h calcination time show good enhancement in its electrical conductivity (8.2 × 101 S m<sup>−</sup><sup>1</sup> ) due to the availability of more surface electrons of rGO. This is best-reported values for conductivity. After hot press process, there is a wide increase in electrical conductivity values when there is a decrease of calcination temperature from 3 to 1-h processing time in these hybrids. Further, the thermal conductivity of γ-Al2O3-rGO is enhanced by more than 80% compared to that of bare γ-Al2O3 when there is an increase in rGO content up to 7.705 wt.% in γ-Al2O3 and rGO hybrids. There is a 77% increase in thermal conductivity using this solvothermal method in these γ-Al2O3 and rGO hybrids. Physical properties such as the BET surface area and bulk density are also improved. Elongated dimensions of nanorods are a major cause of higher mechanical strength in these hybrids. Dielectric constant increases by four orders of magnitude through second percolation threshold with the addition of small rGO in a hybrid. Enhancement in physical properties can be due to well aligned, elongated and fine nanorods morphology of alumina in hybrids, calcination, and hot press processing further played an important role by sustaining quality rGO in hybrids. These nanohybrids of alumina monoliths and rGO can be further applied as catalysts, electrolytes, and as electrochemically active materials because of their nanometer dimensions and enhanced physical properties.
