**3.6 Porosity**

Porosity or void fraction is a measure of void spaces in a material, and is the ratio of the volume of all the pores in a material to the total volume. Radiography of specimen MAS-G4 and MAS-G8 was conducted in order to observe the porosity. Fig. 12 and Fig.13 show radiographic studies for the estimation of porosity. Porosity is the result of gas entrapment in the solidifying glass ceramic materials. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Radiographic images as indicated in Fig. 12b and Fig.13b do not show any dark round or irregular spots or specks and do not predict any sign of porosity. It means that the specimens MAS-G8 and MAS-G4 have through porosity. However, some surface porosity was observed by SEM in both specimens MAS-G8 and MAS-G4 as shown in Fig. 14(a-b).

The particle size distribution curve of specimen MAS-G8 powder is shown in Fig. 11. This curve revealed that the particle size distribution is < 10μm with median particle size is around 5.3μm. Smaller particle size and particle size distribution in narrow range is essential for good sinterability. Experimental results proved the same. It is also found that specimen MAS-G8 sintered at temperature 1040oC provided the material of smaller and uniform particle distribution and density ~2.35 g/cm3 (93% of theoretical density) having 3- 4% through porosity. Because of larger surface area of fine powder, the progress of sintering process accelerates even at low temperatures. However, fine particles, higher the tendency to form agglomerates retarding densification significantly (Ting, & R. Y. Lin, 1995). During pressing, it was observed that if the initial charge was not pulverized for sufficiently long time after calcination, the sintered briquette had a lot of open pores on the surface. So, in order to minimize the porosity in the sintered product, quite long period was used for

> **0.1 0.2 0.4 0.6 0.8 1 1.5 2 3 4 6 8 12 16 24 32 48 Particle Size/micron**

Fig. 11. Particle size distribution of magnesium aluminum silicate glass ceramic MAS-G8

Porosity or void fraction is a measure of void spaces in a material, and is the ratio of the volume of all the pores in a material to the total volume. Radiography of specimen MAS-G4 and MAS-G8 was conducted in order to observe the porosity. Fig. 12 and Fig.13 show radiographic studies for the estimation of porosity. Porosity is the result of gas entrapment in the solidifying glass ceramic materials. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Radiographic images as indicated in Fig. 12b and Fig.13b do not show any dark round or irregular spots or specks and do not predict any sign of porosity. It means that the specimens MAS-G8 and MAS-G4 have through porosity. However, some surface porosity was

observed by SEM in both specimens MAS-G8 and MAS-G4 as shown in Fig. 14(a-b).

**0**

**20**

**40**

**60**

**Histogram/%**

**80**

**100**

**120**

**3.5 Particle size distribution** 

pulverization and finally 72 h time duration was selected.

powder.

**3.6 Porosity** 

**Volume/%**

Fig. 12. (a-b): Radiographic studies of sintered specimens of magnesium aluminum silicate glass ceramic: (a) sintered MAS-G4 (b) Radiographic image of MAS-G4.

Fig. 13. (a-b): Radiographic studies of sintered specimen of magnesium aluminum silicate glass ceramic (a) sintered MAS-G8) (b) Radiographic image of MAS-G8.

Fig. 14. (a-b): SEM image of magnesium aluminum silicate glass ceramic (a) MAS-G8 and (b) MAS-G4.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 271

fluorophlogopite appears as result of different chemical reactions during firing in some sintered MAS specimens. The relative densities (after sintering at 1040-1060oC) of MAS glass specimens were found to be decreased (85-74%) with increase of stoichiometric compositions of K2CO3. These stoichiometric variations altered the density, porosity and other physical properties in agreement with microstructure analyzed by SEM already shown in Fig.14(a-b), where a high fraction of porosity was observed in MAS-G10 specimen.

MAS glass ceramic specimen (MAS-K6) was examined through cooling and quenching after high temperature sintering treatment in order to observed the influence of cooling rate on the development of phases, morphology (grain boundaries ) and their impact on physical properties by studying the process of crystallization. The MAS glass ceramic (MAS-K6) specimen was heated to 1040oC, held for 4h and then cooled. The heating and cooling rates were kept 5oC/min and 5-10oC/min respectively. XRD was used to detect the developed phases on different cooing time. Figure 15 shows the XRD patterns of MAS specimen MAS-K6 (fully cooled down). The XRD data revealed that fluorophlogopite, leucite, and sillimanite are the main phases developed in the sintered bodies along with some other crystalline phases i.e. Mg2SiO4 and SiO2. The observed physical characteristics, morphology and phase evolution are given in **Table 6.** It was noticed that in addition, the glassy phase, fluorophlogopite appears as result of different chemical reactions during firing and cooling. The change in density and other physical appearance were also observed due to different cooling temperatures as shown in Fig.16 (a-d). When the MAS glass material was rapid quenched after sintering exhibit silicarich glassy phases at grain boundaries with enrichment of sillimanite, Mg2SiO4 and less amount of fluorophlogopite, leucite phases, Fig.16 a & f and the surface was found to be porous due to the evaporation of absorbed water. When the MAS material specimen was slowly cooled in furnace, the silica glassy phases were only present along with enrichment of

**3.9 Influence of cooling rate after high temperature sintering treatment** 

fluorophlogopite and leucite phases, Fig.16 b& c.

**Dielectric ~ 1.02**

**1 100 10000 1000000 1E8**

**(a)**

Fig. 16. Impedance analysis of MAS-G8 specimen sintered at 1040oC, (a) Variation of real part of impedance as a function of frequency in Hz (b) Impedance plane plot of MAS-G8

Z// (Ohms)

**0.00E+000 -2.00E+008 -4.00E+008 -6.00E+008 -8.00E+008 -1.00E+009 -1.20E+009 -1.40E+009 -1.60E+009 -1.80E+009 -2.00E+009**

**0.00E+000 6.00E+008 1.20E+009 1.80E+009 2.40E+009 3.00E+009**

**Resistance = 2.72 x 109**

Ω

**(b)**

Z/ (Ohms)

**Frequency (Hz)**

**1**

specimen at RT.

**10**

**Real party of permittivity**

**100**

#### **3.7 Resistance measurement**

The impedance plane plots of MAS glass specimens sintered at 1040 and 1050oC at room temperature are shown in Fig.15 and Fig.16. The Fig.15b and Fig.16b show the typical impedance plane plots between real (Z/) and imaginary parts (Z//) of impedance. An arc-like behavior was noticed in both the plots and the intersection of the arc at low frequency (right hand side) gives the total resistance of the specimens , whereas, intersection at high frequency (the extension of arc on the left hand side) passes through origin. When MAS-G4 glass specimen heated to 1050oC, the intersection of arc on the right hand side shows a decrease in the magnitude of Z/, when sintering temperature is increased from 1040oC to 1450oC. This decrease can be explained as heating will cause grain growth which results decrease in impedance values of the specimen. The impedance values calculated from Fig.15b and Fig.16b, revealed high magnitude of resistance i.e., 1.28 x 109 and 2.72 x 109 ohms for MAS-G4 and MAS-G8 specimens respectively. Real part of permittivity (dielectric) shows the dispersion at low frequencies but at high frequency the dielectric value is least around 1.0 in both MAS glass ceramic specimens, which is according to literature (Hu et.al. 2001). It was observed that MAS-G8 glass specimen is more resistive and insulator in nature than the MAS-G4 and these result values confirmed the XRD, SEM and radiographic results of same sintered specimens.

Fig. 15. Impedance analysis of MAS-G4 specimen sintered at 1050oC, (a) Variation of real part of impedance as a function of frequency in Hz (b) Impedance plane plot of MAS-G4 specimen at RT.

#### **3.8 Effect of K2CO3 concentration**

The physico-chemical characteristics and phase evolution during firing of MAS-K series ceramic bodies was investigated as a function of K2CO3 concentration and firing temperatures. The physico-chemical characteristics of sintered MAS glass specimens with variation of K2CO3 compositions expressed as MAS-K1- MAS-K10 and are presented in **Table 5**. Figure 15 shows the XRD patterns of MAS specimens MAS-K2-MAS-K5. The phase evolution examined by X-ray diffraction (XRD) revealed that leucite, and sillimanite are the main phases in the sintered bodies. Besides the above mentioned crystalline phases, other crystalline Mg2SiO4 and SiO2 phases are also identified in Fig.16. In addition, glassy phase

The impedance plane plots of MAS glass specimens sintered at 1040 and 1050oC at room temperature are shown in Fig.15 and Fig.16. The Fig.15b and Fig.16b show the typical impedance plane plots between real (Z/) and imaginary parts (Z//) of impedance. An arc-like behavior was noticed in both the plots and the intersection of the arc at low frequency (right hand side) gives the total resistance of the specimens , whereas, intersection at high frequency (the extension of arc on the left hand side) passes through origin. When MAS-G4 glass specimen heated to 1050oC, the intersection of arc on the right hand side shows a decrease in the magnitude of Z/, when sintering temperature is increased from 1040oC to 1450oC. This decrease can be explained as heating will cause grain growth which results decrease in impedance values of the specimen. The impedance values calculated from Fig.15b and Fig.16b, revealed high magnitude of resistance i.e., 1.28 x 109 and 2.72 x 109 ohms for MAS-G4 and MAS-G8 specimens respectively. Real part of permittivity (dielectric) shows the dispersion at low frequencies but at high frequency the dielectric value is least around 1.0 in both MAS glass ceramic specimens, which is according to literature (Hu et.al. 2001). It was observed that MAS-G8 glass specimen is more resistive and insulator in nature than the MAS-G4 and these result

values confirmed the XRD, SEM and radiographic results of same sintered specimens.

**(a)**

Fig. 15. Impedance analysis of MAS-G4 specimen sintered at 1050oC, (a) Variation of real part of impedance as a function of frequency in Hz (b) Impedance plane plot of MAS-G4

The physico-chemical characteristics and phase evolution during firing of MAS-K series ceramic bodies was investigated as a function of K2CO3 concentration and firing temperatures. The physico-chemical characteristics of sintered MAS glass specimens with variation of K2CO3 compositions expressed as MAS-K1- MAS-K10 and are presented in **Table 5**. Figure 15 shows the XRD patterns of MAS specimens MAS-K2-MAS-K5. The phase evolution examined by X-ray diffraction (XRD) revealed that leucite, and sillimanite are the main phases in the sintered bodies. Besides the above mentioned crystalline phases, other crystalline Mg2SiO4 and SiO2 phases are also identified in Fig.16. In addition, glassy phase

**0.00E+000 -1.00E+008 -2.00E+008 -3.00E+008 -4.00E+008 -5.00E+008 -6.00E+008 -7.00E+008 -8.00E+008**

**Z// (Ohms)**

**0.00E+000 3.00E+008 6.00E+008 9.00E+008 1.20E+009**

**Resistance = 1.28 x 109**

Ω

**(b)**

**Z / (Ohms)**

**1 100 10000 1000000 1E8**

**Frequency (Hz)**

 **Dielectric ~ 1.02**

**1**

specimen at RT.

**3.8 Effect of K2CO3 concentration** 

**10**

**Real party of permittivity**

**100**

**1000**

**3.7 Resistance measurement** 

fluorophlogopite appears as result of different chemical reactions during firing in some sintered MAS specimens. The relative densities (after sintering at 1040-1060oC) of MAS glass specimens were found to be decreased (85-74%) with increase of stoichiometric compositions of K2CO3. These stoichiometric variations altered the density, porosity and other physical properties in agreement with microstructure analyzed by SEM already shown in Fig.14(a-b), where a high fraction of porosity was observed in MAS-G10 specimen.

#### **3.9 Influence of cooling rate after high temperature sintering treatment**

MAS glass ceramic specimen (MAS-K6) was examined through cooling and quenching after high temperature sintering treatment in order to observed the influence of cooling rate on the development of phases, morphology (grain boundaries ) and their impact on physical properties by studying the process of crystallization. The MAS glass ceramic (MAS-K6) specimen was heated to 1040oC, held for 4h and then cooled. The heating and cooling rates were kept 5oC/min and 5-10oC/min respectively. XRD was used to detect the developed phases on different cooing time. Figure 15 shows the XRD patterns of MAS specimen MAS-K6 (fully cooled down). The XRD data revealed that fluorophlogopite, leucite, and sillimanite are the main phases developed in the sintered bodies along with some other crystalline phases i.e. Mg2SiO4 and SiO2. The observed physical characteristics, morphology and phase evolution are given in **Table 6.** It was noticed that in addition, the glassy phase, fluorophlogopite appears as result of different chemical reactions during firing and cooling. The change in density and other physical appearance were also observed due to different cooling temperatures as shown in Fig.16 (a-d). When the MAS glass material was rapid quenched after sintering exhibit silicarich glassy phases at grain boundaries with enrichment of sillimanite, Mg2SiO4 and less amount of fluorophlogopite, leucite phases, Fig.16 a & f and the surface was found to be porous due to the evaporation of absorbed water. When the MAS material specimen was slowly cooled in furnace, the silica glassy phases were only present along with enrichment of fluorophlogopite and leucite phases, Fig.16 b& c.

Fig. 16. Impedance analysis of MAS-G8 specimen sintered at 1040oC, (a) Variation of real part of impedance as a function of frequency in Hz (b) Impedance plane plot of MAS-G8 specimen at RT.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 273

Specimens collected after quenching (cooling rate 10oC/min).

1040 740 440 140 30

F= 47, L= 12 S= 37 O= 4

Broken , hard and brittle

Off-white and shining with less porosity

Figure 16f Figure 16e Figure 16d Figure 16c Figure 16b

F= 50, L= 8 S= 37 O= 5

hard and to some extent Machinable

White with porosity (5-

6%

After thermal treatment, specimen was remained in furnace in respective time interval. Cooling at furnace rate contemplates cutting off the electric current to the furnace and allowing the furnace to cool down for desire time and also until room temperature.

F= 52, L= 4 S= 38 O= 4

Machinable

White with porosity (5- 6%)

Sintering temperature =1040oC

Density (g/cm3) 2.05 2.19 2.34 2.31 2.28

Broken , hard and brittle

Deformed grey-white color with more porosity

F= Fluorophlogopite L= Leucite (L) S = Sillimanite O= other phases like Mg2SiO4 and SiO2

Table 6. Influence of cooling rate on MAS glass ceramic specimen (MAS-K6) after high

The sintered specimens were tested for resistance to acids and bases when subjected to 5% hydrochloric acid, hydrofluoric acid at 95oC for 24h for acids and 6h for sodium hydroxide and sodium carbonate bases **Table 7.** The chemical resistance i.e. weight loss per unit area (mg/cm2) of MAS-4b either acids or base was found significant and values were found

F= 42, L= 18 S= 31 O= 9

Physico-chemical properties

Phase formation

F= 25, L= 21 S= 48 O= 6

and brittle

Deformed, grey white color with more porosity

After thermal treatment, specimen was taken out from the furnace and quenched in air until room temperature

Characteristic Swell up, hard

(%)

Physical appearance (Shape)

Withdrawal of specimen conditions

Photographs Figure 16(a-f)

**3.11 Acid effect** 

temperature sintering treatment.

similar with (Wawrziniak et al., 1980).


F= Fluorophlogopite L= Leucite (L) S = Sillimanite O= other phases like Mg2SiO4 and SiO2

Table 5. Effect of K2CO3 concentration on sintered density, phase purity and physical properties of MAS glass ceramic materials.

### **3.10 Elemental contents**

The elemental composition of prepared powder and sintered specimens of MAS-G8 glass ceramic was measured by electron probe micro analyzer (EPMA) attached with scanning electron microscope and these elemental contents were also identified by XRF. Fig. 17(a-b) shows the composition analysis of MAS-G8 specimen sintered at 1040oC estimated by EPMA and also analyzed on XRF in order to detect the contents Al, Mg, K and Si.


F= Fluorophlogopite L= Leucite (L) S = Sillimanite O= other phases like Mg2SiO4 and SiO2

Table 6. Influence of cooling rate on MAS glass ceramic specimen (MAS-K6) after high temperature sintering treatment.

### **3.11 Acid effect**

272 Sintering of Ceramics – New Emerging Techniques

Temperature oC

SiO2 Al2O3 MgO K2CO3 B2O3 MgF2 FLSO

K1 43.8 17.6 16.5 12.3 5.8 4 1015 2.42 38 15 42 5 Brittle

K2 35.6 22.5 18.7 12.6 5.8 4.8 1020 2.32 34 25 29 12 Brittle and

K3 47.2 12.6 16.9 14.7 5.9 4.5 1025 2.27 36 44 14 3 Brittle and

K4 40.2 15.5 17.9 15.4 5.7 5.3 1030 2.34 42 12 37 9 Hard and

K5 36.4 24.7 13.5 11.4 3.6 10.4 1040 2.38 57 3 40 - Mechinable

K6 45.3 12.4 17.1 13.7 6.8 4.7 1040 2.39 52 39 9 - Mechinable

K7 45.5 16.8 15.9 12.6 4 5.2 1040 2.28 52 4 38 4 Mechinable

K8 37.5 20.2 17.7 12.8 5 6.8 1045 2.41 54 5 39 2 Mechinable

K9 37.5 20.2 17.7 12.8 5 6.8 1050 2.41 51 7 42 - Mechinable

K10 38.5 16.4 19.4 11.6 7.5 6.6 1045 2.41 49 36 7 8 Hard and

K11 36.2 23.6 16.1 9.8 4 10.3 1070 2.37 46 5 40 9 Hard and

The elemental composition of prepared powder and sintered specimens of MAS-G8 glass ceramic was measured by electron probe micro analyzer (EPMA) attached with scanning electron microscope and these elemental contents were also identified by XRF. Fig. 17(a-b) shows the composition analysis of MAS-G8 specimen sintered at 1040oC estimated by

F= Fluorophlogopite L= Leucite (L) S = Sillimanite O= other phases like Mg2SiO4 and SiO2

EPMA and also analyzed on XRF in order to detect the contents Al, Mg, K and Si.

properties of MAS glass ceramic materials.

**3.10 Elemental contents** 

Table 5. Effect of K2CO3 concentration on sintered density, phase purity and physical

Sintered Density (g/cc)

XRD

(%Phase) Properties

deformed

swell up

Brittle

Brittle

Brittle

(Wt %) Firing

Chemical Composition

Specimen #

MAS-

MAS-

MAS-

MAS -

MAS-

MAS-

MAS-

MAS-

MAS-

MAS-

MAS -

The sintered specimens were tested for resistance to acids and bases when subjected to 5% hydrochloric acid, hydrofluoric acid at 95oC for 24h for acids and 6h for sodium hydroxide and sodium carbonate bases **Table 7.** The chemical resistance i.e. weight loss per unit area (mg/cm2) of MAS-4b either acids or base was found significant and values were found similar with (Wawrziniak et al., 1980).

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 275

**0**

**20**

**40**

Intensity (a.u.)

Fig. 19. (a-b): Chemical composition analysis of MAS-G8 (a) EPMA spectra (b) XRF patterns.

HF (5%) (mg/cm2)

MAS- G4 95 24 67 17.51 95 6 10.62 Nil

MAS- G6 95 24 87 156 95 6 9.52 2.13

MAS- G8 95 24 48 6 95 6 11.82 1.35

Table 7. Acids and bases effect on magnesium aluminum silicate glass ceramic specimens.

On the basis of experiments performed and capability for preparation of MAS glass ceramic materials using sintering route with 93% relative density contain 57% fluorophlogopite phase responsible for machinability. The high voltage insulators are required indifferent shapes and sizes so for these reasons some MAS glass ceramic materials have been developed by sintering route in different shape and size. The photographs of developed

Temper ature oC

Time (h)

Resistance to Acid Wt-loss per unit area

> HCl (5%) (mg/cm2)

Time (h)

MAS glass ceramic items are as shown in Fig. 20.

Specimen #

Temperat ure oC

**60**

**80**

**100**

**150 135 120 105 90 75 60 45**

Si

**2** θ **(degree)**

Resistance to Base Wt-loss per unit area

> NaOH (5%) (mg/cm2)

Na2CO3 (5%) (mg/cm2)

2 θ = 107.4

2 θ = 136.9

Al

Mg

2 θ = 141.6

2 θ = 52.6

K

Fig. 17. XRD patterns of MAS glass ceramic specimens (MAS-K1-MASK5) as a function of variation of K2CO3 concentration and firing temperatures.

Fig. 18. (a-f): SEM image and photographs of MAS glass ceramic as a function of cooling rates after high sintering temperature (a) SEM image of MAS-K6 specimen sintered at 1040oC and cooled at 30oC (b) MAS-K6 specimen sintered at 1040oC and cooled at 30oC (c) sintered at 1040oC and cooled at 140oC (d) sintered at 1040oC and cooled at 440oC (e) sintered at 1040oC and cooled at 740oC (f) sintered at 1040oC and cooled at 1040oC.

**10 20 30 40 50 60 70 80 90**

K6

K5

K4

K3

K2

**2** θ **(degree)**

Fig. 17. XRD patterns of MAS glass ceramic specimens (MAS-K1-MASK5) as a function of

Fig. 18. (a-f): SEM image and photographs of MAS glass ceramic as a function of cooling rates after high sintering temperature (a) SEM image of MAS-K6 specimen sintered at 1040oC and cooled at 30oC (b) MAS-K6 specimen sintered at 1040oC and cooled at 30oC (c) sintered at 1040oC and cooled at 140oC (d) sintered at 1040oC and cooled at 440oC (e) sintered at 1040oC and cooled at 740oC (f) sintered at 1040oC and cooled at 1040oC.

variation of K2CO3 concentration and firing temperatures.

Intensity (a.u)

Fig. 19. (a-b): Chemical composition analysis of MAS-G8 (a) EPMA spectra (b) XRF patterns.


Table 7. Acids and bases effect on magnesium aluminum silicate glass ceramic specimens.

On the basis of experiments performed and capability for preparation of MAS glass ceramic materials using sintering route with 93% relative density contain 57% fluorophlogopite phase responsible for machinability. The high voltage insulators are required indifferent shapes and sizes so for these reasons some MAS glass ceramic materials have been developed by sintering route in different shape and size. The photographs of developed MAS glass ceramic items are as shown in Fig. 20.

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005521-1

Fig. 20. (a-c): MAS glass ceramic materials in different shape and size (a) showing solid slab, cylindrical rod, green crucible, disc (b) green and sintered slabs (c) sintered rod.
