**4. Summary of results and discussion**

As we have already noted, the innovative point of current research is, besides the use of the applied method of sinter-crystallization toward the production of sintered glass-ceramics and/or glass-ceramic foams, the use of a controlled environment during the synthesis of sintered glass-ceramics and in particular extended to the design and production of glass-ceramic foams.

In **Figure 1**, a comparative graphical representation is presented, together with images at respective temperatures of interest, of the shape alteration during realtime in situ filming of two samples being synthesized in air or in argon atmosphere.

Up to temperatures of about 1050°C, the thermal behavior of both species is quite comparable. Both samples undergo similar densification during the interval of low temperature sintering. At higher temperatures however, iron-rich glass-ceramic materials heated up in different (air or inert) atmosphere reveal a completely different thermal behavior compared to each other when they are subject to a subsequent thermal treatment.

In the case of an argon-sintered sample, further heating of the material leads to sample melting (to, e.g., a hemispheric spill at 1155°C). However, the sample sintered in air starts expanding its volume during heat treatment above 1050°C, and at the same reference temperature of 1155°C, it reveals a maximal value of the structural expansion determined by gas evolution—the foaming in the entire bulk of the newly formed material. This difference in the thermal behavior of both samples is mainly due to differences in the respective material's viscosity values.

The phenomena described above are entirely determined by the state of the redox couple equilibrium Fe(II) ↔ Fe(III) in a sense that an inert environment (e.g., argon) is going to maintain the equilibrium drawn at a maximal extent to the left (oxidation is inhibited [9]), while the air atmosphere is going to keep the ratio Fe(II)/Fe(III) at a minimal value, i.e., the equilibrium is drawn to the right.

In **Figure 2a**, the sintering curves are presented during an isothermal scan and the respective low temperature behavior of the glass-ceramic samples in air and argon. One can clearly and unambiguously note the effect of the environment on sintering: the onset of the sintering process in argon is shifted to lower temperatures. It starts earlier, and the degree of structural densification is to a certain extent higher than the one in air (14 vs. 12% shrinkage, c.f. again **Figure 2a**). The shaded area in **Figure 2a** represents actually the area of temperatures where the glass transition point, *Tg*, which is the most important physical characteristic of a glass, is to be observed.

In **Figure 2a**, the shaded area reveals in fact, as it is obvious in **Figure 2b**, that an ODLT measurement represents certainly a method for truly reliable measurements

**77**

**Figure 1.**

observed as well.

argon amounts to Δ*T* = 20°C.

*Sintered Iron-Rich Glass-Ceramics and Foams Obtained in Air and Argon*

of the temperature of vitrification, *Tg*, of pressed powder objects as well. It is evident that the glass transition point of the sample sintered in argon is significantly diminished and the difference in *Tg* between glass-ceramics synthesized in air and

*Typical HSM curves of the thermal behavior of pressed iron-rich glass-ceramic samples in air and argon.*

One can thus summarize that utilizing the ODLT technique for the sake of synthesis of well-sintered glass-ceramic materials is a good approach. It also reveals the possibility for reliable measurements of *Tg*, due to its high precision as a result of lack of mechanical parts, and for exact tuning of the appropriate firing regimes. XRD spectra of the glass-ceramic foams obtained in air and argon environment are presented in **Figure 3**. The phase analysis of the samples results in the detection of pyroxene crystal phases. The crystallinity of the samples is between 30 and 40% as referred also by Strnad [21]. The crystallinity in argon however is higher than that in air. A shift of the peak positions in air to lower theta angles has been

*DOI: http://dx.doi.org/10.5772/intechopen.88941*

*Sintered Iron-Rich Glass-Ceramics and Foams Obtained in Air and Argon DOI: http://dx.doi.org/10.5772/intechopen.88941*

*Foams - Emerging Technologies*

**4. Summary of results and discussion**

thermal treatment.

the design and production of glass-ceramic foams.

Scanning electron microscopy (SEM) was used to analyze the structure of the sintered glass-ceramics by taking pictures of both fractures and the surfaces of the samples. A JEOL 6390 (Japan) instrument was used. To provide electron conductiv-

3-D computed micro-tomography was used for entire bulk scanning of the foam glass-ceramic species. The tomographic measurements were carried out with an X-ray micro-tomograph, Bruker SKYSCAN 1272 (Germany), which uses a white beam with cone geometry. The following setup conditions were applied: X-ray tube voltage 70 kV, current 142 mA and 0.11 mm Cu filter. The voxel (3-D pixel) size was 1 μm and the optical magnification was 7.4. A typical 360° scan took 21 hours and 27 minutes. Reconstruction of the 3-D images was done with the commercial software InstaRecon. The phase composition of the sintered glass-ceramic foams was determined by X-ray

diffraction spectroscopy (XRD) using a Panalytical Empyrean (USA) spectrometer.

As we have already noted, the innovative point of current research is, besides the use of the applied method of sinter-crystallization toward the production of sintered glass-ceramics and/or glass-ceramic foams, the use of a controlled environment during the synthesis of sintered glass-ceramics and in particular extended to

In **Figure 1**, a comparative graphical representation is presented, together with images at respective temperatures of interest, of the shape alteration during realtime in situ filming of two samples being synthesized in air or in argon atmosphere. Up to temperatures of about 1050°C, the thermal behavior of both species is quite comparable. Both samples undergo similar densification during the interval of low temperature sintering. At higher temperatures however, iron-rich glass-ceramic materials heated up in different (air or inert) atmosphere reveal a completely different thermal behavior compared to each other when they are subject to a subsequent

In the case of an argon-sintered sample, further heating of the material leads to sample melting (to, e.g., a hemispheric spill at 1155°C). However, the sample sintered in air starts expanding its volume during heat treatment above 1050°C, and at the same reference temperature of 1155°C, it reveals a maximal value of the structural expansion determined by gas evolution—the foaming in the entire bulk of the newly formed material. This difference in the thermal behavior of both samples is

The phenomena described above are entirely determined by the state of the redox couple equilibrium Fe(II) ↔ Fe(III) in a sense that an inert environment (e.g., argon) is going to maintain the equilibrium drawn at a maximal extent to the left (oxidation is inhibited [9]), while the air atmosphere is going to keep the ratio Fe(II)/Fe(III) at a minimal value, i.e., the equilibrium is drawn to the right.

In **Figure 2a**, the sintering curves are presented during an isothermal scan and the respective low temperature behavior of the glass-ceramic samples in air and argon. One can clearly and unambiguously note the effect of the environment on sintering: the onset of the sintering process in argon is shifted to lower temperatures. It starts earlier, and the degree of structural densification is to a certain extent higher than the one in air (14 vs. 12% shrinkage, c.f. again **Figure 2a**). The shaded area in **Figure 2a** represents actually the area of temperatures where the glass transition point, *Tg*, which is the most important physical characteristic of a glass, is to be observed.

In **Figure 2a**, the shaded area reveals in fact, as it is obvious in **Figure 2b**, that an ODLT measurement represents certainly a method for truly reliable measurements

mainly due to differences in the respective material's viscosity values.

ity, all samples were metalized with gold by vapor deposition technique.

**76**

of the temperature of vitrification, *Tg*, of pressed powder objects as well. It is evident that the glass transition point of the sample sintered in argon is significantly diminished and the difference in *Tg* between glass-ceramics synthesized in air and argon amounts to Δ*T* = 20°C.

One can thus summarize that utilizing the ODLT technique for the sake of synthesis of well-sintered glass-ceramic materials is a good approach. It also reveals the possibility for reliable measurements of *Tg*, due to its high precision as a result of lack of mechanical parts, and for exact tuning of the appropriate firing regimes.

XRD spectra of the glass-ceramic foams obtained in air and argon environment are presented in **Figure 3**. The phase analysis of the samples results in the detection of pyroxene crystal phases. The crystallinity of the samples is between 30 and 40% as referred also by Strnad [21]. The crystallinity in argon however is higher than that in air. A shift of the peak positions in air to lower theta angles has been observed as well.

**Figure 2.**

*(a) Sintering curves measured by ODLT in air and argon. (b) Sintering curves; zoom-in of the shaded area of (a). The glass transition temperature (Tg) is unambiguously to be recognized in both curves.*

### **Figure 3.**

*XRD spectra of glass-ceramic foams synthesized in different environment.*

The two features described above confirm that the absence of oxidation in argon environment leads unambiguously to higher crystallinity and to some differences in the chemical composition of the pyroxene phases. These differences can hypothetically be due to the facilitated entering of the Fe(II) and Mn(III) ions in the pyroxene structures. Also a change in the lattice interplanar distance is present.

In order for the microstructure of the sintered glass-ceramic species subject to current investigation to be studied, a series of scanning electron microscope images of two sintered glass-ceramic samples are shown in **Figure 4**. Images have been taken from the surface and from a fracture of both species. **Figure 4a** and **c** represents photos of the surface of the air-sintered sample; **Figure 4b** and **d** is from the surface of the argon-sintered sample; **Figure 4e**, **g** and **h** is images from a fracture of the airsintered sample; and **Figure 4f** is a picture of a fracture of the argon-sintered sample.

Despite the well overall sintering in both atmospheres, it has been found that in argon environment an even better sintering of the sample than that in air is present.

**79**

**Figure 4.**

*SEM images from a fracture and the surface of two sintered glass-ceramic foam samples in air and argon: 4a and 4c: surface of the air sintered sample; 4e, 4g, 4h: fracture of the air sintered sample. 4b and 4d: surface of* 

*the argon sintered sample; 4f: fracture of the argon sintered sample.*

*Sintered Iron-Rich Glass-Ceramics and Foams Obtained in Air and Argon*

*DOI: http://dx.doi.org/10.5772/intechopen.88941*

*Sintered Iron-Rich Glass-Ceramics and Foams Obtained in Air and Argon DOI: http://dx.doi.org/10.5772/intechopen.88941*

*Foams - Emerging Technologies*

**Figure 2.**

**Figure 3.**

**78**

The two features described above confirm that the absence of oxidation in argon environment leads unambiguously to higher crystallinity and to some differences in the chemical composition of the pyroxene phases. These differences can hypothetically be due to the facilitated entering of the Fe(II) and Mn(III) ions in the pyroxene

*(a) Sintering curves measured by ODLT in air and argon. (b) Sintering curves; zoom-in of the shaded area of* 

*(a). The glass transition temperature (Tg) is unambiguously to be recognized in both curves.*

In order for the microstructure of the sintered glass-ceramic species subject to current investigation to be studied, a series of scanning electron microscope images of two sintered glass-ceramic samples are shown in **Figure 4**. Images have been taken from the surface and from a fracture of both species. **Figure 4a** and **c** represents photos of the surface of the air-sintered sample; **Figure 4b** and **d** is from the surface of the argon-sintered sample; **Figure 4e**, **g** and **h** is images from a fracture of the airsintered sample; and **Figure 4f** is a picture of a fracture of the argon-sintered sample. Despite the well overall sintering in both atmospheres, it has been found that in argon environment an even better sintering of the sample than that in air is present.

structures. Also a change in the lattice interplanar distance is present.

*XRD spectra of glass-ceramic foams synthesized in different environment.*

### **Figure 4.**

*SEM images from a fracture and the surface of two sintered glass-ceramic foam samples in air and argon: 4a and 4c: surface of the air sintered sample; 4e, 4g, 4h: fracture of the air sintered sample. 4b and 4d: surface of the argon sintered sample; 4f: fracture of the argon sintered sample.*

### *Foams - Emerging Technologies*

An interesting observation is that on the surface of the argon-sintered glassceramics (**Figure 4b**), the pores reveal a concave morphology. This might be an indication for the formation of these pores in the interval of softening which initiates together with the beginning of the process of high-temperature reduction as well. In both cases of sintering, a large population of small spherical holes on the surface is present, which is almost a certain indication for gas release.

In both cases of sintering, a non-spherical, sharp-edge intergranular porosity is predominant.

The proposed possibility for a selective environment of synthesis of sintered glass-ceramics and glass-ceramic foams provides the option of the synthesis to be carried out and thus to be experimented by using different stages.

Thus after a low temperature sintering stage in air (e.g., see **Figure 1**), the process of foaming of a pressed glass sample can be carried out in a second stage

**Figure 5.**

*Measurements of the thermal shape alteration behavior during foaming in argon by HSM; snapshots of the sample at a respective characteristic temperature.*

**81**

**Figure 6.**

*Sintered Iron-Rich Glass-Ceramics and Foams Obtained in Air and Argon*

entirely in argon environment. The process of foaming of this sample traced by a HSM measurement is shown in **Figure 5**. The snapshots in **Figure 5** during the run of a thermal scan represent the evolution of structural alteration at charac-

In order for the effect of the atmosphere on the foaming of sintered glassceramics to be carefully analyzed, a graphical plot of the structural evolution during foaming in different atmospheres is given in **Figure 6**. The plot in the expansiontime domain of foaming in air and argon during an isothermal scan at a temperature of 1100°C with 30 minutes holding time (**Figure 6**) provides a clear and detailed

The expansion of the structure due to the release of oxygen gas and formation of the closed-cell system during high-temperature reduction initiates in both environments (air and inert) in a similar way and almost at the same time as well. The process of foaming in argon however turns to be more effective. The bloating of the material reaches a maximal value of nearly 200%, considerably earlier than the maximal foaming in air (up to 190%); then after a slight volume shrinkage, a stable material with 180% expanded structure is being formed and maintained in the

The entire foam material formation (i.e., the formation of closed porosity population) proceeds relatively quickly with the programmed isotherm in **Figure 6**. The foaming in argon takes ~5 minutes to complete followed by a small shrinkage and a structural stabilization. The foaming in air takes longer time to complete than argon (15 minutes) and reaches stabilization again. The foam material formation in air and argon atmospheres results in obtaining a new material characterized in both

For the sake of investigation of the entire bulk structure of the newly formed foam material, 3-D X-ray tomographic analysis has been used. In **Figure 7a** (left), a false-color 3-D reconstruction of the surface is presented and the volume by tomographic scanning of an iron-rich glass-ceramic foam sample synthesized in air. In **Figure 7a** (right), a selection of three cross sections of the bulk of the same sample is presented. In **Figure 7b** (left), similar 3-D false-color surface-volume reconstruction of a glass-ceramic foam sample synthesized in argon is presented. In **Figure 7b**

cases generally with fire resistance properties at temperatures of 1100°C.

(right), cross-section slices of the material's bulk are shown, respectively.

*Temporal evolution of foaming curves in air and argon of sintered glass-ceramic samples.*

*DOI: http://dx.doi.org/10.5772/intechopen.88941*

picture on the process under consideration.

course of the working isotherm.

teristic temperatures.

*Foams - Emerging Technologies*

predominant.

An interesting observation is that on the surface of the argon-sintered glassceramics (**Figure 4b**), the pores reveal a concave morphology. This might be an indication for the formation of these pores in the interval of softening which initiates together with the beginning of the process of high-temperature reduction as well. In both cases of sintering, a large population of small spherical holes on the

In both cases of sintering, a non-spherical, sharp-edge intergranular porosity is

The proposed possibility for a selective environment of synthesis of sintered glass-ceramics and glass-ceramic foams provides the option of the synthesis to be

Thus after a low temperature sintering stage in air (e.g., see **Figure 1**), the process of foaming of a pressed glass sample can be carried out in a second stage

*Measurements of the thermal shape alteration behavior during foaming in argon by HSM; snapshots of the* 

surface is present, which is almost a certain indication for gas release.

carried out and thus to be experimented by using different stages.

**80**

**Figure 5.**

*sample at a respective characteristic temperature.*

entirely in argon environment. The process of foaming of this sample traced by a HSM measurement is shown in **Figure 5**. The snapshots in **Figure 5** during the run of a thermal scan represent the evolution of structural alteration at characteristic temperatures.

In order for the effect of the atmosphere on the foaming of sintered glassceramics to be carefully analyzed, a graphical plot of the structural evolution during foaming in different atmospheres is given in **Figure 6**. The plot in the expansiontime domain of foaming in air and argon during an isothermal scan at a temperature of 1100°C with 30 minutes holding time (**Figure 6**) provides a clear and detailed picture on the process under consideration.

The expansion of the structure due to the release of oxygen gas and formation of the closed-cell system during high-temperature reduction initiates in both environments (air and inert) in a similar way and almost at the same time as well. The process of foaming in argon however turns to be more effective. The bloating of the material reaches a maximal value of nearly 200%, considerably earlier than the maximal foaming in air (up to 190%); then after a slight volume shrinkage, a stable material with 180% expanded structure is being formed and maintained in the course of the working isotherm.

The entire foam material formation (i.e., the formation of closed porosity population) proceeds relatively quickly with the programmed isotherm in **Figure 6**. The foaming in argon takes ~5 minutes to complete followed by a small shrinkage and a structural stabilization. The foaming in air takes longer time to complete than argon (15 minutes) and reaches stabilization again. The foam material formation in air and argon atmospheres results in obtaining a new material characterized in both cases generally with fire resistance properties at temperatures of 1100°C.

For the sake of investigation of the entire bulk structure of the newly formed foam material, 3-D X-ray tomographic analysis has been used. In **Figure 7a** (left), a false-color 3-D reconstruction of the surface is presented and the volume by tomographic scanning of an iron-rich glass-ceramic foam sample synthesized in air. In **Figure 7a** (right), a selection of three cross sections of the bulk of the same sample is presented. In **Figure 7b** (left), similar 3-D false-color surface-volume reconstruction of a glass-ceramic foam sample synthesized in argon is presented. In **Figure 7b** (right), cross-section slices of the material's bulk are shown, respectively.

**Figure 6.** *Temporal evolution of foaming curves in air and argon of sintered glass-ceramic samples.*

### **Figure 7.**

*X-ray computed tomography, (a) 3-D reconstruction of the surface and the bulk of a glass-ceramic foam synthesized in air (left); volume cross sections (right), and (b) 3-D reconstruction of the surface and the bulk of a glass-ceramic foam synthesized in argon (left); volume cross sections (right).*

From visual observations and the performed analysis, one can clearly note that the porosity in the bulk of both samples is predominantly of closed type and amounts to about 80–85% in both species. It is also evident that the walls of the samples are abundant of pores. Another interesting feature is that the closed cells of the species foamed in air are larger than the cells in the volume of the sample foamed in argon. Moreover the thickness of the walls in the sample synthesized in air is on average lower than the wall thickness in the glass-ceramic foam material obtained in argon environment.
