**3. Results and discussion**

#### **3.1 XRD phase analysis**

Experimental results showed the phase stability, thermal stability, compressibility, and sinterability of MAS glass ceramic materials. The crystallinity of MAS glass was studied by

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 263

**0 10 20 30 40 50 60 70 80**

**2** θ **(degree)**

Fig. 5. XRD patterns of magnesium aluminum silicate glass ceramic, specimen MAS-G3,

The TG/DTA analysis is proper method to determine the optimal stabilization temperature **(**Perdomol, et al., 1998; & Izquierdo-Barba, et al., 1999). Thermal analysis TG, DTA of selected specimen the MAS-G8 was carried in the temperature range of 40-1000oC in order to find the crystallization temperature of different crystalline phases as shown in Fig. 6. The total weight loss 9.14% was observed after heating up to 900oC. Since the specimen was not fully dried, some physically bounded water was present at surface of crystals and in micropores, which caused a subsequent loss of mass at the lowest temperature range, i.e., 50-250oC. This process is endothermic as confirmed by DTA. Two peaks in the DTA data were observed. The first peak is endothermic, which is connected with decomposition of water vapors and glass transformation. It can be seen that crystal phase (nucleation) occurs at nearly 732oC, which is evident from the large exothermic peak. The exothermal maxima correspond to the separation of the crystalline phase. The position of the minimum on DTA curve is basically determined by the chemical composition of the separated phase or by its transformation temperature. The exothermal crystallization peak, its position and shape have been characterized by the crystallization process **(**Bapna & Mueller, H. 1996). In crystallization process [K2O-Al2O3-SiO2-MgO-B2O3-F], first crystal phase to appear is the aluminium borate-mullite solid solution crystal which on increasing temperature, transforms to fluorophlogopite on reaction with the matrix phase **(**Roy, S. et al. (2011). Heating rate during DTA is around 10oC/min. All the volatiles were completely removed at 900oC. The nucleation temperature of this crystal is a strong function of heating rate as found by (Bapna & Mueller, H. 1996) and observed that with increasing heating rate shifts the nucleation temperature to a higher value. The percent weight loss of other specimens

**MAS-G3 51%(F) 49% (S)**

**MAS-G4 56%(F) 43% (S)**

**MAS-G8 57%(F) 43% (S)**

**0**

MAS-G4 and MAS-G8 along with standard.

MAS-G2 to MAS-10 is shown in Table 4.

**100**

**200**

**300**

**400**

**Intensity (a.u.)**

**3.2 Thermal analysis** 

**500**

**600**

**700**

XRD and XRF. The XRD patterns of specimens MAS-G3, MAS-G4 and MAS-G8 sintered at 1040°C for 3h along with standard specimens are presented in Fig. 5. The XRD patterns were indexed and compared with Joint Committee of Powder Diffraction Standard (JCPDS) data cards (McClune, W. F.1989). The predominant phases in the prepared MAS specimens were identified as fluorophlogopite (potassium magnesium aluminum silicate, KMg3AlSi3O10F2, JPCPD # 71-1542), sillimanite (aluminum silicon oxide, Al2SiO5, JPCPD #.88-0893) and leucite (potassium aluminum silicate KAlSi2O6 JPCPD # 15-0047). The percentage of these phases in each specimen is given in **Table 2,** whereas crystallographic XRD analysis data are presented in **Table 3.** The well formed diffraction patterns and "d" values confirmed that the predominant crystal phase in MAS-G8 is 57% fluorophlogopite and this fluorophlogopite crystal phase was found near to standard values i.e., 60% fluorophlogopite. It was also observed that specimen MAS-G8 consists of 57%fluorophlogopite crystal phase due to which it possess very good mechinability, but very difficult to be sintered to fully dense state. The leucite (potassium-aluminum-silicate, KAlSi2O6) crystal phase with large coefficient of thermal expansion is formed when glass ceramic is heated and held at temperatures between 1025°C and 1530°C.


F= Fluorophlogopite L= Leucite and S= Sillimanite

Table 2. Fluorophlogopite phase of magnesium aluminum silicate glass ceramics with different composition.


Table 3. XRD phase analysis of MAS-G8 specimen.

Fig. 5. XRD patterns of magnesium aluminum silicate glass ceramic, specimen MAS-G3, MAS-G4 and MAS-G8 along with standard.

#### **3.2 Thermal analysis**

262 Sintering of Ceramics – New Emerging Techniques

XRD and XRF. The XRD patterns of specimens MAS-G3, MAS-G4 and MAS-G8 sintered at 1040°C for 3h along with standard specimens are presented in Fig. 5. The XRD patterns were indexed and compared with Joint Committee of Powder Diffraction Standard (JCPDS) data cards (McClune, W. F.1989). The predominant phases in the prepared MAS specimens were identified as fluorophlogopite (potassium magnesium aluminum silicate, KMg3AlSi3O10F2, JPCPD # 71-1542), sillimanite (aluminum silicon oxide, Al2SiO5, JPCPD #.88-0893) and leucite (potassium aluminum silicate KAlSi2O6 JPCPD # 15-0047). The percentage of these phases in each specimen is given in **Table 2,** whereas crystallographic XRD analysis data are presented in **Table 3.** The well formed diffraction patterns and "d" values confirmed that the predominant crystal phase in MAS-G8 is 57% fluorophlogopite and this fluorophlogopite crystal phase was found near to standard values i.e., 60% fluorophlogopite. It was also observed that specimen MAS-G8 consists of 57%fluorophlogopite crystal phase due to which it possess very good mechinability, but very difficult to be sintered to fully dense state. The leucite (potassium-aluminum-silicate, KAlSi2O6) crystal phase with large coefficient of thermal expansion is formed when glass ceramic is heated and held at temperatures between 1025°C

Sintering Conditions Density (g/cm3) Phase (%) by

(h) Green Sintered

Nucleation XRD

Time

MAS-G1 630 1040 3 1.79 2.18 F= 28 L= 71 MAS-G2 630 1040 3 1.74 2.12 F= 30 L = 69 MAS-G3 630 1040 3 1.91 2.29 F=51 S= 49 MAS-G4 630 1050 3 1.68 2.15 F = 56 S = 43 MAS-G5 630 1040 3 1.65 2.12 F= 28 L= 71 MAS-G6 630 1050 3 1.72 2.14 F=49 S= 42 MAS-G7 630 1060 4 1.64 2.14 F = 50 S = 44 MAS-G8 630 1040 3 1.81 2.38 F= 57 S = 43 MAS-G9 630 1070 2 1.86 2.39 F= 54 S = 42 MAS-G10 630 1080 2 1.74 2.35 F= 51 S = 39 Standard 2.48 F= 60 S = 40

Table 2. Fluorophlogopite phase of magnesium aluminum silicate glass ceramics with

Crystallographic XRD analysis Fluorophlogopite Silliminite

Unit cell volume(Å3) 481.77 331.23 Crystallite size (Å) 184 184 Lattice parameters a (Å) 5.308 7.49 b (Å) 9.183 7.67 c (Å) 10.00 5.77

Symmetry Monoclinic Orthorhombic

and 1530°C.

Specimen #

(oC)

F= Fluorophlogopite L= Leucite and S= Sillimanite

Table 3. XRD phase analysis of MAS-G8 specimen.

different composition.

Temperature (oC)

> The TG/DTA analysis is proper method to determine the optimal stabilization temperature **(**Perdomol, et al., 1998; & Izquierdo-Barba, et al., 1999). Thermal analysis TG, DTA of selected specimen the MAS-G8 was carried in the temperature range of 40-1000oC in order to find the crystallization temperature of different crystalline phases as shown in Fig. 6. The total weight loss 9.14% was observed after heating up to 900oC. Since the specimen was not fully dried, some physically bounded water was present at surface of crystals and in micropores, which caused a subsequent loss of mass at the lowest temperature range, i.e., 50-250oC. This process is endothermic as confirmed by DTA. Two peaks in the DTA data were observed. The first peak is endothermic, which is connected with decomposition of water vapors and glass transformation. It can be seen that crystal phase (nucleation) occurs at nearly 732oC, which is evident from the large exothermic peak. The exothermal maxima correspond to the separation of the crystalline phase. The position of the minimum on DTA curve is basically determined by the chemical composition of the separated phase or by its transformation temperature. The exothermal crystallization peak, its position and shape have been characterized by the crystallization process **(**Bapna & Mueller, H. 1996). In crystallization process [K2O-Al2O3-SiO2-MgO-B2O3-F], first crystal phase to appear is the aluminium borate-mullite solid solution crystal which on increasing temperature, transforms to fluorophlogopite on reaction with the matrix phase **(**Roy, S. et al. (2011). Heating rate during DTA is around 10oC/min. All the volatiles were completely removed at 900oC. The nucleation temperature of this crystal is a strong function of heating rate as found by (Bapna & Mueller, H. 1996) and observed that with increasing heating rate shifts the nucleation temperature to a higher value. The percent weight loss of other specimens MAS-G2 to MAS-10 is shown in Table 4.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 265

fluorophlogopite phase. Thermal expansion co-efficient (alpha) of MAS-G8 ceramic was measured at different temperature ranges by dilatometric technique. The coefficient of thermal expansion (α = x10-5 oC-1) was measured using a dilatometer Fig. 7(a-b) and Fig. 8(ab) in air at 1000oC for specimens MAS-4 and MAS-G8; the values are -1.563 and -1.093 x10-5 oC-1 respectively. The result revealed that α decrease with increase of chemical composition. The detailed coefficient of thermal expansion (α = x10-5 oC-1) values are presented in **Table 4.** 

**0 200 400 600 800 1000 1200**

**Nucleation** 

**Temperature o**

Physico-chemical Properties

Weight loss (DG/DTA)

Thermal co-efficient (200-1100 oC)

Median diameter of particle

**C**

Table 4. Physical properties of MAS glass ceramics.

**980 oC**

**Growth**

**Sintering** 

**(a)**

compact of magnesium aluminum silicate glass ceramic specimen MAS-G4.

Fig. 7. (a-b): Thermodilatometric (a) and differential dilatometric (b) curves of green

**-40**

Property Units MAS-G2 MAS-G4 MAS-G6 MAS-G8 MAS-

Micro hardness Vicker 449 484 504 527 505

Resistivity Ω 1.07 x 109 1.27 x 109 1.35 x 109 2.72 x 109 2.05 x 109

Porosity % 7-8 7 5 2-3 2-3

Theoretical density % 79 87 81 93 91

% 10.15 10.42 9.05 9.14 9.32

oC-1 -1.563x10-5 -1.563x10-5 -1.563x10-5 -1.093x10-5 -1.063x10-5

μm 4.4 5.4 5.6 5.3 4.9

**-30**

**-20**

**-10**

Differential dimensional change(dl/LO\*10-2

)

**208.5 o C**

**0**

**10**

**0 200 400 600 800 1000 1200**

**684.9 oC**

C

**965.2 o C** **(b)**

G10

Temperature o

**551.8 o C**

**-10**

**-8**

**-6**

**-4**

**Dimension Change (dl/Lo\*10-2)**

**-2**

**0**

**2**

Fig. 6. TG/DTA curves for magnesium aluminum silicate glass ceramic powder, specimen MAS-G8.

#### **3.3 Green compact and thermo dilatometric study**

The behavior of densification was measured by dilatometry in order to understand what is occurring during sintering. Dilatometry is a well-known method of studying densification kinetics during the sintering process of ceramic bodies **(**Holkova*, et al.,* 2003). In dilatometry, the compacted ceramic body undergoes heat treatment in the dilatometer to initiate sintering. Simultaneously, the length of the compacted body is measured as a function of time at a given temperature. The densification can further be assessed by measurement of post-sintering density, whereby as the compacted body shrinks, its density will increase. Isothermal sintering experiments were carried out by heating samples rapidly after binder burnout to the sintering temperature and the data collected at intervals that are set in the control program. An entire densification profile can be obtained from a single sample. The characteristics of pressed powder of MAS-G4 and MAS-G8 during sintering were examined by means of dilatometry. Fig. 7 and Fig.8 display the thermo-dilatometric curves of MAS-G4 and MAS-G8 powders which were calcined at 650°C and then pressed into green pellet respectively. The curve shows a simple thermal shirking or expansion i.e., the change in length during sintering of pellet. There was no change in length on heating up to temperature of 208°C but started at about 557oC. However, a large shrinkage occurred as a result of evolution of decomposed species between 300 and 400°C. The temperature between 400 and 700°C shows the expansion in the material. This expansion is due to nucleation process i.e. decomposition of meat-stable phases of magnesium and aluminum silicates and carbonates and fluorides also the expulsion of organic binder (PVA). Above 850°C, there is gradual increase in dimensional changes (lattice parameters) which are due to crystal growth (development of MAS-G8 crystalline phase). On the basis of dilatometry results, sintering was conducted between 985-1040°C in order to obtain the crystalline MAS-G8

**200 400 600 800 1000**

**C**

**Exothermic**

**-30**

**-25**

**-20**

**-15**

**-10**

**Temperature Difference /uV** 

**-5**

**0**

**5**

**10**

**882 o C**

**Temperature/ o**

Fig. 6. TG/DTA curves for magnesium aluminum silicate glass ceramic powder, specimen

The behavior of densification was measured by dilatometry in order to understand what is occurring during sintering. Dilatometry is a well-known method of studying densification kinetics during the sintering process of ceramic bodies **(**Holkova*, et al.,* 2003). In dilatometry, the compacted ceramic body undergoes heat treatment in the dilatometer to initiate sintering. Simultaneously, the length of the compacted body is measured as a function of time at a given temperature. The densification can further be assessed by measurement of post-sintering density, whereby as the compacted body shrinks, its density will increase. Isothermal sintering experiments were carried out by heating samples rapidly after binder burnout to the sintering temperature and the data collected at intervals that are set in the control program. An entire densification profile can be obtained from a single sample. The characteristics of pressed powder of MAS-G4 and MAS-G8 during sintering were examined by means of dilatometry. Fig. 7 and Fig.8 display the thermo-dilatometric curves of MAS-G4 and MAS-G8 powders which were calcined at 650°C and then pressed into green pellet respectively. The curve shows a simple thermal shirking or expansion i.e., the change in length during sintering of pellet. There was no change in length on heating up to temperature of 208°C but started at about 557oC. However, a large shrinkage occurred as a result of evolution of decomposed species between 300 and 400°C. The temperature between 400 and 700°C shows the expansion in the material. This expansion is due to nucleation process i.e. decomposition of meat-stable phases of magnesium and aluminum silicates and carbonates and fluorides also the expulsion of organic binder (PVA). Above 850°C, there is gradual increase in dimensional changes (lattice parameters) which are due to crystal growth (development of MAS-G8 crystalline phase). On the basis of dilatometry results, sintering was conducted between 985-1040°C in order to obtain the crystalline MAS-G8

**-21**

**-18**

**-15**

**-12**

**Mass Loss / %**

MAS-G8.

**-9**

**-6**

**TG**

**Mass loss = 9.14%**

**DTA**

**3.3 Green compact and thermo dilatometric study** 

**-3**

fluorophlogopite phase. Thermal expansion co-efficient (alpha) of MAS-G8 ceramic was measured at different temperature ranges by dilatometric technique. The coefficient of thermal expansion (α = x10-5 oC-1) was measured using a dilatometer Fig. 7(a-b) and Fig. 8(ab) in air at 1000oC for specimens MAS-4 and MAS-G8; the values are -1.563 and -1.093 x10-5 oC-1 respectively. The result revealed that α decrease with increase of chemical composition. The detailed coefficient of thermal expansion (α = x10-5 oC-1) values are presented in **Table 4.** 

Fig. 7. (a-b): Thermodilatometric (a) and differential dilatometric (b) curves of green compact of magnesium aluminum silicate glass ceramic specimen MAS-G4.


Table 4. Physical properties of MAS glass ceramics.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 267

Fig. 9. (a-c): The effect of heating temperature and time duration on physical surface of sintered specimens (a) Open pores on the surface of MAS-G4 (b) Swelling of MAS material

Fig. 10. (a-c): Photograph of MAS-G4 glass specimen (a) green and sintered specimens (b) microscopic image and (c) SEM micrograph of polished sintered MAS-G4 glass specimen.

(c) brittle and broken of sintered slab.

Fig. 8. (a-b): Thermodilatometric (a) and differential dilatometric (b) curves of green compact of magnesium aluminum silicate glass ceramic specimen MAS-G8.

#### **3.4 Sintering MAS glass ceramic**

Green solid briquette of dimensions (61x16x5mm) was obtained using hydraulic press. The green body exhibits significant changes of its properties resulting from dehydration at low temperatures, phase changes during dehydroxlation, high-temperature reactions, and densification during sintering. All these changes significantly influence phyisco-mechanical properties of the fired body.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 and MAS glass ceramic material looks swelled and deformed as shown in Fig. 9(a-c). The percent relative densities were measured in order to evaluate the material performance. The effect of heating temperature and time duration on density of sintered specimens MAS-G6 to MAS-G10 as a function of sintering temperature is shown in **Table 2**. Insignificant increase in density was observed in sintering temperature range 1040oC to 1060oC with time duration range 2-4h. It was observed that as the sintering temperature was increased beyond 1060oC, excessive thermal energy leads to some rearrangement among the grains as well as some an isotropic crystal growth. In addition, decomposition of fluorophlogopite phase, formed at lower temperature, takes place leading to the development of internal line cracks/voids (Radojic, & Nikolic, 1991) in the material. As a result of this, the material looks swelled, broken, deformed and its density decreases at higher sintering temperatures, Fig. 9(a-c). The important sintering parameters, green and sintered densities of samples MAS-G1 to MAS-G10 are given in **Table 2.** Relative density (93%) was obtained, when the MAS-8 glass ceramic specimen was sintered at 1040oC for 4h. The high sintered density can be attributed due to small degree of agglomeration and fine nature of powder. The results revealed that sintered MAS-G8 glass ceramic is composed of uniformly sized grains and is >93% of theoretical density. Formation of fluorophlogopite phase, good relative density and micro porosity are mainly responsible for producing machinability properties. Fig.10 (a-b) shows photographic, microscopic and scanning electron image of green and sintered MAS-G4 glass specimens whereas XRD pattern of such specimen is shown in Fig. 5.

**-20**

Green solid briquette of dimensions (61x16x5mm) was obtained using hydraulic press. The green body exhibits significant changes of its properties resulting from dehydration at low temperatures, phase changes during dehydroxlation, high-temperature reactions, and densification during sintering. All these changes significantly influence phyisco-mechanical properties of the fired body.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 and MAS glass ceramic material looks swelled and deformed as shown in Fig. 9(a-c). The percent relative densities were measured in order to evaluate the material performance. The effect of heating temperature and time duration on density of sintered specimens MAS-G6 to MAS-G10 as a function of sintering temperature is shown in **Table 2**. Insignificant increase in density was observed in sintering temperature range 1040oC to 1060oC with time duration range 2-4h. It was observed that as the sintering temperature was increased beyond 1060oC, excessive thermal energy leads to some rearrangement among the grains as well as some an isotropic crystal growth. In addition, decomposition of fluorophlogopite phase, formed at lower temperature, takes place leading to the development of internal line cracks/voids (Radojic, & Nikolic, 1991) in the material. As a result of this, the material looks swelled, broken, deformed and its density decreases at higher sintering temperatures, Fig. 9(a-c). The important sintering parameters, green and sintered densities of samples MAS-G1 to MAS-G10 are given in **Table 2.** Relative density (93%) was obtained, when the MAS-8 glass ceramic specimen was sintered at 1040oC for 4h. The high sintered density can be attributed due to small degree of agglomeration and fine nature of powder. The results revealed that sintered MAS-G8 glass ceramic is composed of uniformly sized grains and is >93% of theoretical density. Formation of fluorophlogopite phase, good relative density and micro porosity are mainly responsible for producing machinability properties. Fig.10 (a-b) shows photographic, microscopic and scanning electron image of green and sintered MAS-

**-15**

**-10**

Differential dimension change(dl/Lo\*10-2

Fig. 8. (a-b): Thermodilatometric (a) and differential dilatometric (b) curves of green

compact of magnesium aluminum silicate glass ceramic specimen MAS-G8.

G4 glass specimens whereas XRD pattern of such specimen is shown in Fig. 5.

)

**-5**

**0**

**0 200 400 600 800 1000 1200**

C

**(b)**

**731o C**

**1008 o C**

Temperature <sup>o</sup>

**614 o C**

**0 200 400 600 800 1000 1200**

**Nucleation**

**Crystallization**

Sintering **(a)**

**Temperature o C**

**3.4 Sintering MAS glass ceramic** 

**-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4**

Dimension Change (dL/LO\*10-2)

Fig. 9. (a-c): The effect of heating temperature and time duration on physical surface of sintered specimens (a) Open pores on the surface of MAS-G4 (b) Swelling of MAS material (c) brittle and broken of sintered slab.

Fig. 10. (a-c): Photograph of MAS-G4 glass specimen (a) green and sintered specimens (b) microscopic image and (c) SEM micrograph of polished sintered MAS-G4 glass specimen.

Synthesis and Sintering Studies of Magnesium Aluminum Silicate Glass Ceramic 269

 Fig. 12. (a-b): Radiographic studies of sintered specimens of magnesium aluminum silicate

 Fig. 13. (a-b): Radiographic studies of sintered specimen of magnesium aluminum silicate

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

glass ceramic: (a) sintered MAS-G4 (b) Radiographic image of MAS-G4.

glass ceramic (a) sintered MAS-G8) (b) Radiographic image of MAS-G8.

(b) MAS-G4.

#### **3.5 Particle size distribution**

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 pulverization and finally 72 h time duration was selected.

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