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Phase separation, crystallization and leaching of microporous glass ceramics in the CaO–TiO2–P2O5 system
Journal of Non-Crystalline Solids 355 (2009) 141–147
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Phase separation, crystallization and leaching of microporous glass ceramics in the CaO–TiO2–P2O5 system M. Kord, V.K. Marghussian *, B. Eftekhari-yekta, A. Bahrami Ceramics Div., Dept. of Materials, Iran University of Science and Technology, Narmak, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 26 September 2008 Available online 25 November 2008 Keywords: Crystallization Glass ceramics Nucleation Microstructure Nanocrystals
a b s t r a c t Microporous glass ceramics belonging to the CaO–TiO2–P2O5 system were prepared with the assumption of a 2:1 mole ratio for b-Ca3(PO4)2:CaTi4(PO4)6, the anticipated crystalline phases in the end product. The glasses formulated according to the above composition were melted and cast onto a steel mold and were crystallized to glass ceramics containing the above phases. Dilatometric/differential thermal analysis (DTA) techniques were utilized to determine the appropriate phase separation–nucleation and crystallization temperatures. The crystalline products and resulting microstructures in various stages of process were determined and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By leaching the resulting glass ceramics in HCl, b-Ca3(PO4)2 was dissolved out leaving a porous skeleton of CaTi4(PO4)6. It was found that the volume porosity, specific surface area and mean pore diameter of microporous glass ceramics can be managed through the proper selection of heat treatment conditions. In the optimized conditions for fabricating glass ceramics of minimum mean pore size the values of 41 ± 4%, 26 ± 3 m2/g and 14.3 ± 2 nm were obtained for porosity, surface area and pore diameter respectively. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Porous ceramics have attracted considerable attention in recent years, and are currently used in various engineering applications as membranes [1], supporting materials for immobilization of enzymes [2], catalyst supports [3] and sensors [4]. On the other hand microporous glass ceramic materials, have many advantages over conventional porous ceramics such as a more convenient production technology, better control over the size and distribution of pores and the possibility of producing crystalline skeletons of differing compositions, structures and functionalities. These materials have also some advantages in comparison with the earlier Vycor porous glasses [5], regarding the chemical and mechanical properties, easier production routes and more versatility in the skeletal material properties. The porous glass ceramics are usually prepared utilizing phosphate glasses which are mainly located in CaO–TiO2–P2O5 or related systems. The glasses after undergoing a spinodal type phase separation (like Vycor glasses) are heat treated to develop crystalline phases such as b-Ca3(PO4)2 and CaTi4(PO4)6. The former phase is usually leached out, leaving behind a porous skeleton of CaTi4(PO4)6 [6–8]. * Corresponding author. Tel.: +98 21 73912814; fax: +98 21 77240480. E-mail address: [email protected] (V.K. Marghussian). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.10.006
Although many technical aspects of the process including the fabrication, details of crystallization, acid leaching and effect of composition changes on properties has been investigated and reported in literature [6–11], some aspects of the whole process have remained not fully explained and unclear, e.g. despite the marked effect of the occurrence of glass-in glass phase separation by spinodal decomposition mechanism upon the process, it has never substantiated experimentally, the scientific basis for the determination of nucleation and crystallization temperatures and using unusually long holding times at these temperatures are unclear, the details of crystallization process has not been explained, the microstructural studies are rare and the effect of temperature and time on the leaching of glass ceramics has not been studied. In this paper an effort was made to clarify and describe some aspects of the production processes, especially the phase separation, crystallization and leaching, in a more comprehensive manner. 2. Experimental procedure The studied glass composition, formulated with a b-Ca3(PO4)3:CaTi4(PO4)6 mole ratio of 2:1, is presented in Table 1.The raw materials used to make the glass were regent grade CaCO3,TiO2 and P2O5. The glasses were melted in alumina crucibles at 1350 °C in an electric furnace for 2 h and were cast onto preheated steel moulds. The glass specimens then were annealed at 600 °C for
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2 h and furnace cooled. The glass specimens after crystallization heat treatments were leached in 1 M HCl solution in order to dissolve the more soluble, b-Ca3(PO4)2 phase and to produce a porous skeleton of CaTi4(PO4)6. Differential thermal analysis (DTA) were performed on the glass specimens (polymer laboratories STA-1640) using samples of <65 lm and 0.5–0.6 mm particle size ranges in order to determine the crystallization temperature and the susceptibility to bulk crystallization.In DTA runs alumina was used as an inert reference material and the heating rate was 10 °C min 1. Glass transition temperature (Tg) and dilatometric softening point (Td) were also determined by dilatometer (Netzsch 402E) and used to demarcate the glass-in glass phase separation and nucleation temperature ranges. Three specimens were used for each DTA test. X-ray diffraction (XRD) was used in order to identify the crystallization products in heat treated specimens (Siemens-D500). Cu Ka radiation was used in XRD examinations in the 2h range of 10– 100°. The samples after polishing and etching were coated with a thin film of gold and subjected to SEM examination (scanning electron microscope, Philips-XL-30). Some fractured surfaces of porous specimens were also examined. The specific surface area and the mean pore size of porous specimens were measured utilizing the Nitrogen adsorption and desorption technique (Micromeritics-2000, ASAP).Three separate specimens were used for the test.
3. Results 3.1. Phase separation As the annealed glass specimens had not shown clearly observable phase separation, even in very high magnification SEM examination, it was decided to carry out a heat treatment operation in order to initiate or intensify the separation process in glass
Table 1 Nominal chemical composition of the glass specimen. Oxide
P2O5
CaO
TiO2
Mol%
31.25
43.75
25
Fig. 2. DTA traces for glass specimens of different particle sizes (a) <75 lm, (b) <0.5– 0.6 mm.
Fig. 1. SEM micrographs showing phase separation in glass specimen after heating at 710 °C (a) 6 h, (b) 18 h, (c) 24 h.
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Fig. 3. XRD patterns for specimens heat treated at differing temperatures for 4 h after nucleation at 710 °C for 24 h (a) 745 °C, (b) 755 °C, (c) 765 °C.
specimens. Dilatometric technique was used to determine the suitable heat treatment range for the phase separation process which usually occurs in glass transition (Tg)-dilatometric softening temperature (Td) interval. The above temperatures were determined as 700 ± 3 and 720 ± 3 °C for Tg and Td, respectively. SEM micrographs showed phase separation in glass specimens after heating at 710 °C for differing times (Fig. 1(a), (b) and (c). The above temperature was chosen at the middle of Tg and Ts interval. Obviously after a 6 h hold at 710 °C the signs of phase separation are barely observable, whereas after 18 h a very fine microstructure revealing the occurrence of a phase separation with an apparently spinodal decomposition mechanism is observed which after a soaking time of 24 h became quite distinct. 3.2. Crystallization Fig. 2 shows DTA traces for specimens possessing fine and coarse particle sizes. The DTA exothermic peaks usually correspond
Fig. 4. XRD patterns for specimens nucleated at 710 °C for differing times (a–d) followed by a heat treatment at 765 °C for 48 h (a) 6 h, (b) 12 h, (c) 18 h, (d) 24 h.
to crystallization in glass specimens. The difference of 50 °C between the two first peak temperatures of DTA curves i.e. 790 ± 4 and 840 ± 4, is an indication of the relative importance of surface crystallization in these specimens. The much smaller second DTA exo-peaks are related to Ca2 P2O7 and TiO2 irrelevant to the present study. The preference of bulk crystallization in this work necessitated the use of bulk specimens and the selection of the corresponding exo-peak temperature of 840 °C.
Fig. 5. SEM micrographs of specimens nucleated at 710 °C for (a) 12 h and (b) 24 h followed by a heat treatment at 765 °C for 48 h.
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In order to follow the crystallization process in these glass specimens, first the suitable nucleation temperature and time should be found. Since specimens had usually been soaked at a temperature around 700 °C for relatively long times for ‘nucleation’ purposes in previous investigations [6–8], it was supposed that the occurrence of an effective nucleation in these specimens required a precursor phase separation process to occur prior to the actual nucleation. Hence, first a tentative ‘nucleation’ heat treatment at 710 °C for 24 h, the condition of occurrence of distinct phase separation, were considered for the nucleation process. Since the specimens heated at DTA exo-peak temperature of 840 °C subsequent to the nucleation treatment, developed very coarse microstructures it was decided to choose a lower temperature interval of Tc (Tc 50) for crystallization. Where Tc is DTA exothermic peak onset temperature determined as 785 °C for the coarse particle size specimen from DTA curve (Fig. 2) In order to find the suitable crystallization temperature, specimens first were subjected to the aforementioned ‘nucleation’ treatment, and then soaked for 4 h at differing crystallization temperatures in the above interval. Fig. 3 depicts the XRD patterns of some specimens heat treated according to the above procedure. Since the microstructures of glass ceramics controls the pore sizes of final products, in order to keep the pore diameters in the as low as possible range the lowest temperature showing distinct crystallization in XRD examinations, i.e. 765 °C, was chosen as the ‘crystallization’ temperature. In order to assess the actual occurrence of nucleation in the above conditions, the glass specimens after a nucleation treatment at 710 °C for differing times (6–24 h), were heated at 765 °C for 48 h and then subjected to XRD examination. Fig. 4 represents the XRD patterns of the above specimens. It can be seen that all XRD peaks for the major phases b-Ca3(PO4)2 and CaTi4(PO4)6 and the minor phase Ca(PO3)2 have continuously been intensified by prolongation of the nucleation time up to 24 h. On the other hand the SEM microstructures of two specimens nucleated for 24 and 12 h in the above conditions (Fig. 5) show that the specimen held at the nucleation temperature for 24 h developed a much finer microstructure in comparison with the specimen soaked for 12 h, indicating the formation of much greater population of nuclei in the former specimen. Moreover, in order to determine the optimum crystallization time for the specimens, they were first nucleated for 24 h at 710 °C and subsequently soaked for differing times at 765 °C. Fig. 6 shows the XRD patterns for the above specimens. Although it seems that the crystallization products continuously increased from 8 to 48 h the difference shown between 24 and 48 h is not great. On the other hand the microstructural examination by SEM revealed the occurrence of considerable growth and coarsening of crystals in the period of 24–48 h (Fig. 7). Therefore in order to keep the pore sizes in the as low as possible range, it was decided to choose 24 h as the holding time for crystal growth. In order to follow the steps of crystallization process, it was decided to carry out some short time heat treatments on the glass specimens. In these treatments the specimens after a 24 h nucleation at 710 °C, were heated at 765 °C for 15, 30, 45, 60, 90 and 120 min. Fig. 8 exhibits the XRD patterns of the heat treated specimens. It can be seen that after 45 min, considerable crystallization has occurred in the glass specimens that continuously increased up to 120 min. It is interesting to note that the type of crystallization products, namely b-Ca3(PO4)2 and CaTi4(PO4)6 as the major phases and Ca(PO3)2 as the minor phase, were not changed throughout these experiments.
Fig. 9 depicts SEM microstructures of some heat treated specimens. The crystals that started to grow after 45 min within the phase separated glass specimen (Fig. 9(c)), were spread rapidly after 60 min (Fig. 9(d)) and began to exhibit different morphologies i.e. fibrous and nearly equiaxed after 90 and 120 min (Fig. 9(e,f)). 3.3. Leaching The leaching of crystallized specimens carried out in 1 M HCl solution at differing temperatures up to 7 days were followed by measuring the bulk densities vs. time. Fig. 10 depicts the results. It can be deduced that the leaching temperature in the range of ambient temperature 60 °C has no marked effect upon the leaching process.
Fig. 6. XRD patterns for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for differing times (a) 8 h, (b) 16 h, (c) 24 h, (d) 32 h, (e) 40 h, (f) 48 h.
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Fig. 7. SEM micrographs for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for (a) 24 h and (b) 48 h.
Fig. 11 shows the microstructure of a highly porous specimen after leaching. Fig. 12 shows the XRD pattern of the porous specimen. It can be seen that CaTi4(PO4)6 and Ca(PO3)2 are the major and minor phases remained after the leaching process and there is no evidence for the presence of b-Ca3(PO4)2 phase. Table 2 summarizes some physical properties of the specimen. 4. Discussion
Fig. 8. XRD patterns for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for differing times (a) 15 min, (b) 30 min, (c) 45 min, (d) 60 min, (e) 90 min, (f) 120 min.
On the basis of the results described above it can be inferred that the most critical step in the preparation of these glass ceramics is the phase separation process. The occurrence of phase separation by an apparently spinodal decomposition mechanism (Fig. 1(c)) in the glass specimen provides the required conditions for the formation of interconnected crystalline collections of main phases,Ca3(PO4)2 and CaTi4(PO4)6 in the respective phase separated regions of glasses enriched in CaO and TiO2 respectively. Without these interconnected phases the whole process of leaching and fabrication of bodies possessing high values of fine nano-size open porosity would be impossible. On the other hand the comparison of the microstructure of a glass ceramic specimen which prior to the crystallization was subjected to a heat treatment at the phase separation temperature of 710 °C for 24 h with a specimen heated at the same temperature for 12 h (Fig. 5), indicates the existence of a much finer microstructure and hence the formation of much greater number of nuclei at the former specimen. This also resulted in much higher bulk crystallization values as proved by XRD patterns shown in Fig. 4. Therefore it can be inferred that an actual nucleation process was progressing along with the glass-in glass phase separation at 710 °C in these glasses. On the other hand the temperature interval between Tg and Td in which the occurrence of a suitable phase separation becomes possible is quite restricted. Lower temperatures would halt the already slow phase separation process and the acceleration of the phase separation process by raising the temperature may be detrimental to whole process, since this may disrupt the interconnected spinodal microstructure and convert it to a microstructure of isolated droplets distributed within a matrix. Hence it can be concluded that the relatively slow phase separation process (Fig. 1) is mainly responsible for the slow nucleation rates in these glass ceramics. The crystallization (growth) temperature is another important factor that requires careful consideration. Although DTA exo-peak temperature of 840 °C (Fig. 2), can be taken as the suitable crystallization temperature, the relatively coarse microstructures developed in
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Fig. 9. SEM micrographs for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for (c) 45 min, (d) 60 min, (e) 90 min, and (f) 120 min.
specimens heated at this temperature necessitated the use of lower temperatures, around the exothermic peak onset temperature (Fig. 2) e.g. 765 °C as described above. The relatively slow growth rate encountered at the chosen temperature in this experiment necessitated the use of long crystallization times, (at least 24 h). Increasing the temperature could lead to considerable coarsening of the crystals and larger pore in the final product. Therefore it seems that in the case of growth process, similar to the nucleation process, a relatively restricted condition exists in respect to the selection of suitable time/temperature for the heat treatment,
although the condition is more flexible in the case of growth. In this case a relatively limited temperature range, located between To, several degrees lower than DTA crystallization temperature of Tc = 840 °C and Tf = 765 °C can be imagined within which the variation of pore size by the variation of crystallization temperature becomes practically possible and useful. The comparison of Fig. 9(f) with Fig. 5(b), showing the crystallization of specimens after 2 and 24 h heat treatment respectively, reveals that the equiaxed particles eventually adopted plate like shapes, whereas the fibrous particles preserved their fibrous or rod-shape morphology. Also the comparison of the microstructure of porous specimens (Fig. 11) with microstructure of crystallized specimens before leaching (Fig. 5(b) or Fig. 7(a)) reveals that the rod-shape particles are those which remained in the porous specimen after the leach-
Fig. 10. Bulk density vs. time curves for leaching glass ceramic specimens at (a) ambient temperature, (b) 40 °C, and (c) 60 °C.
Fig. 11. SEM micrograph showing the porous specimen after leaching (fractured surface).
M. Kord et al. / Journal of Non-Crystalline Solids 355 (2009) 141–147
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ature, as a inherent difficulty of this fabrication method in controlling the characteristics of the resulting porous material through variation of heat treatment conditions it seems that only the compositional changes leading to higher mobility at relatively low temperatures for the ions taking part in phase separation process, with no adverse effect on the spinodal mechanism, could bring about the useful high separation and nucleation rates. The same problem is encountered in growth process. The relatively slow growth rate occurring at the chosen temperature in this experiment necessitated the use of long crystallization times, (at least 24 h). Increasing the temperature usually leads to considerable coarsening of the crystals and larger pores in the final product. This problem probably may also be solved by selecting a suitable composition, as discussed above. Fig. 12. XRD pattern of the glass ceramic specimen after leaching.
Table 2 Physical properties of porous glass ceramics. Bulk density (g/cm3)
Volume porosity (%)
Specific surface area (m2/g)
Average pore diameter (nm)
1.66 ± 0.05
41 ± 4
26 ± 3
14.3 ± 2
ing operation i.e. CaTi4(PO4)6 and the plate like particles are bCa3(PO4)2 crystals which were leached out along with some residual glass phase. Fig. 11 also reveals that the seemingly rod-shape particles of CaTi4 (PO4)6 are in fact comprised of many tiny crystals. This can be explained by the crystallization mechanism exhibited in Fig. 5(a), which is the manifestation of a spherulitic growth. In this mechanism several crystals begin to grow from an identical central nucleus in the form of radiating fibers, which branch in various directions and soon covered with numerous tiny crystallites [12]. In close observation of Fig. 11 it seems that pores seen in this figure, which are mainly in the 70–200 nm diameter range, are much larger than the measured mean pore diameter of 14.3 nm. This can be explained by the existence of numerous fine pores on the external and internal pore walls, which are observable with higher magnifications and is consistent to the findings of other investigators [6]. Although it was not possible to differentiate between the crystallization steps of different phases, from Figs. 9 and 11 it can be inferred that both major phases begin to crystallize simultaneously from different phase separated regions, rich in CaO or TiO2, and remained confined within their respective boundaries to the end of crystallization process. 4.1. Suggestion for further work On the basis of the above discussion regarding the restricted conditions for selection of nucleation and growth time and temper-
5. Conclusions It was shown that glass-in glass phase separation with a spinodal decomposition mechanism is the precursor step for the occurrence of an effective nucleation, in the glass ceramics studied herein. In this way phase separation plays a very decisive role in controlling the crystallization process and affecting the microstructure of the glass ceramics, hence determining the size and vol.% of the final porosity in the end product. The optimum phase separation–nucleation temperature and time determined as 710 °C and 24 h, in fact should be considered as an almost invariable condition for the fabrication of these type of porous materials (if composition was kept constant) with acceptable properties and nanosize porosity. The chosen crystallization (growth) condition of 765 °C and 24 h was also proved to be quite effective in producing bodies of acceptable values of porosity (41 ± 4%) and quite small (perhaps the minimum possible) mean pore diameter of 14.3 ± 2 nm. The obtained glass ceramics after leaching in 1 M HCl solution at the ambient temperature, 40 °C and 60 °C for up to 7 days did not show any considerable differences in the leaching rate or final bulk density and volume porosity values. References [1] T. Yazawa, H. Tanaka, in: K. Ishizaki (Ed.), Porous Materials, American Ceramic Society, Westerville, OH, 1993, p. 213. [2] R.A. Messing, J. Non-Cryst. Solids 26 (1977) 482. [3] I. Aso, M. Nakao, N. Yamazoe, T. Seeiyama, J. Catal. 57 (1979) 287. [4] M. Uo, M. Numata, M. Suzuki, I. Karube, A. Mkishima, J. Ceram. Soc. Jpn. 100 (1992) 430. [5] H.P. Hood, M.E. Nordberg, US patent 2, 106, 774 (1934). [6] H. Hosono, Z. Zhang, Y. Abe, J. Am. Ceram. Soc. 72 (1989) 1587. [7] H. Hosono, Y. Sakai, M. Fasano, Y. Abe, J. Am. Ceram. Soc. 73 (1990) 2536. [8] H. Hosono, Y. Abe, J. Am. Ceram. Soc. 75 (1992) 2862. [9] H. Hosono, Y. Abe, Non-Cryst. 190 (1995) 185. [10] K. Yamamoto, T. Kasuga, Y. Abe, J. Am. Ceram. Soc. 80 (1997) 822. [11] T. Kasuga, M. Nogami, Y. Abe, J. Am. Ceram. Soc. 82 (1999) 765. [12] V.K. Marghussin, A. Arjomandnia, Phys. Chem. Glasses 40 (1999) 311.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Phase separation, crystallization and leaching of microporous glass ceramics in the CaO–TiO2–P2O5 system M. Kord, V.K. Marghussian *, B. Eftekhari-yekta, A. Bahrami Ceramics Div., Dept. of Materials, Iran University of Science and Technology, Narmak, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 25 April 2008 Received in revised form 26 September 2008 Available online 25 November 2008 Keywords: Crystallization Glass ceramics Nucleation Microstructure Nanocrystals
a b s t r a c t Microporous glass ceramics belonging to the CaO–TiO2–P2O5 system were prepared with the assumption of a 2:1 mole ratio for b-Ca3(PO4)2:CaTi4(PO4)6, the anticipated crystalline phases in the end product. The glasses formulated according to the above composition were melted and cast onto a steel mold and were crystallized to glass ceramics containing the above phases. Dilatometric/differential thermal analysis (DTA) techniques were utilized to determine the appropriate phase separation–nucleation and crystallization temperatures. The crystalline products and resulting microstructures in various stages of process were determined and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). By leaching the resulting glass ceramics in HCl, b-Ca3(PO4)2 was dissolved out leaving a porous skeleton of CaTi4(PO4)6. It was found that the volume porosity, specific surface area and mean pore diameter of microporous glass ceramics can be managed through the proper selection of heat treatment conditions. In the optimized conditions for fabricating glass ceramics of minimum mean pore size the values of 41 ± 4%, 26 ± 3 m2/g and 14.3 ± 2 nm were obtained for porosity, surface area and pore diameter respectively. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Porous ceramics have attracted considerable attention in recent years, and are currently used in various engineering applications as membranes [1], supporting materials for immobilization of enzymes [2], catalyst supports [3] and sensors [4]. On the other hand microporous glass ceramic materials, have many advantages over conventional porous ceramics such as a more convenient production technology, better control over the size and distribution of pores and the possibility of producing crystalline skeletons of differing compositions, structures and functionalities. These materials have also some advantages in comparison with the earlier Vycor porous glasses [5], regarding the chemical and mechanical properties, easier production routes and more versatility in the skeletal material properties. The porous glass ceramics are usually prepared utilizing phosphate glasses which are mainly located in CaO–TiO2–P2O5 or related systems. The glasses after undergoing a spinodal type phase separation (like Vycor glasses) are heat treated to develop crystalline phases such as b-Ca3(PO4)2 and CaTi4(PO4)6. The former phase is usually leached out, leaving behind a porous skeleton of CaTi4(PO4)6 [6–8]. * Corresponding author. Tel.: +98 21 73912814; fax: +98 21 77240480. E-mail address: [email protected] (V.K. Marghussian). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.10.006
Although many technical aspects of the process including the fabrication, details of crystallization, acid leaching and effect of composition changes on properties has been investigated and reported in literature [6–11], some aspects of the whole process have remained not fully explained and unclear, e.g. despite the marked effect of the occurrence of glass-in glass phase separation by spinodal decomposition mechanism upon the process, it has never substantiated experimentally, the scientific basis for the determination of nucleation and crystallization temperatures and using unusually long holding times at these temperatures are unclear, the details of crystallization process has not been explained, the microstructural studies are rare and the effect of temperature and time on the leaching of glass ceramics has not been studied. In this paper an effort was made to clarify and describe some aspects of the production processes, especially the phase separation, crystallization and leaching, in a more comprehensive manner. 2. Experimental procedure The studied glass composition, formulated with a b-Ca3(PO4)3:CaTi4(PO4)6 mole ratio of 2:1, is presented in Table 1.The raw materials used to make the glass were regent grade CaCO3,TiO2 and P2O5. The glasses were melted in alumina crucibles at 1350 °C in an electric furnace for 2 h and were cast onto preheated steel moulds. The glass specimens then were annealed at 600 °C for
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2 h and furnace cooled. The glass specimens after crystallization heat treatments were leached in 1 M HCl solution in order to dissolve the more soluble, b-Ca3(PO4)2 phase and to produce a porous skeleton of CaTi4(PO4)6. Differential thermal analysis (DTA) were performed on the glass specimens (polymer laboratories STA-1640) using samples of <65 lm and 0.5–0.6 mm particle size ranges in order to determine the crystallization temperature and the susceptibility to bulk crystallization.In DTA runs alumina was used as an inert reference material and the heating rate was 10 °C min 1. Glass transition temperature (Tg) and dilatometric softening point (Td) were also determined by dilatometer (Netzsch 402E) and used to demarcate the glass-in glass phase separation and nucleation temperature ranges. Three specimens were used for each DTA test. X-ray diffraction (XRD) was used in order to identify the crystallization products in heat treated specimens (Siemens-D500). Cu Ka radiation was used in XRD examinations in the 2h range of 10– 100°. The samples after polishing and etching were coated with a thin film of gold and subjected to SEM examination (scanning electron microscope, Philips-XL-30). Some fractured surfaces of porous specimens were also examined. The specific surface area and the mean pore size of porous specimens were measured utilizing the Nitrogen adsorption and desorption technique (Micromeritics-2000, ASAP).Three separate specimens were used for the test.
3. Results 3.1. Phase separation As the annealed glass specimens had not shown clearly observable phase separation, even in very high magnification SEM examination, it was decided to carry out a heat treatment operation in order to initiate or intensify the separation process in glass
Table 1 Nominal chemical composition of the glass specimen. Oxide
P2O5
CaO
TiO2
Mol%
31.25
43.75
25
Fig. 2. DTA traces for glass specimens of different particle sizes (a) <75 lm, (b) <0.5– 0.6 mm.
Fig. 1. SEM micrographs showing phase separation in glass specimen after heating at 710 °C (a) 6 h, (b) 18 h, (c) 24 h.
M. Kord et al. / Journal of Non-Crystalline Solids 355 (2009) 141–147
143
Fig. 3. XRD patterns for specimens heat treated at differing temperatures for 4 h after nucleation at 710 °C for 24 h (a) 745 °C, (b) 755 °C, (c) 765 °C.
specimens. Dilatometric technique was used to determine the suitable heat treatment range for the phase separation process which usually occurs in glass transition (Tg)-dilatometric softening temperature (Td) interval. The above temperatures were determined as 700 ± 3 and 720 ± 3 °C for Tg and Td, respectively. SEM micrographs showed phase separation in glass specimens after heating at 710 °C for differing times (Fig. 1(a), (b) and (c). The above temperature was chosen at the middle of Tg and Ts interval. Obviously after a 6 h hold at 710 °C the signs of phase separation are barely observable, whereas after 18 h a very fine microstructure revealing the occurrence of a phase separation with an apparently spinodal decomposition mechanism is observed which after a soaking time of 24 h became quite distinct. 3.2. Crystallization Fig. 2 shows DTA traces for specimens possessing fine and coarse particle sizes. The DTA exothermic peaks usually correspond
Fig. 4. XRD patterns for specimens nucleated at 710 °C for differing times (a–d) followed by a heat treatment at 765 °C for 48 h (a) 6 h, (b) 12 h, (c) 18 h, (d) 24 h.
to crystallization in glass specimens. The difference of 50 °C between the two first peak temperatures of DTA curves i.e. 790 ± 4 and 840 ± 4, is an indication of the relative importance of surface crystallization in these specimens. The much smaller second DTA exo-peaks are related to Ca2 P2O7 and TiO2 irrelevant to the present study. The preference of bulk crystallization in this work necessitated the use of bulk specimens and the selection of the corresponding exo-peak temperature of 840 °C.
Fig. 5. SEM micrographs of specimens nucleated at 710 °C for (a) 12 h and (b) 24 h followed by a heat treatment at 765 °C for 48 h.
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In order to follow the crystallization process in these glass specimens, first the suitable nucleation temperature and time should be found. Since specimens had usually been soaked at a temperature around 700 °C for relatively long times for ‘nucleation’ purposes in previous investigations [6–8], it was supposed that the occurrence of an effective nucleation in these specimens required a precursor phase separation process to occur prior to the actual nucleation. Hence, first a tentative ‘nucleation’ heat treatment at 710 °C for 24 h, the condition of occurrence of distinct phase separation, were considered for the nucleation process. Since the specimens heated at DTA exo-peak temperature of 840 °C subsequent to the nucleation treatment, developed very coarse microstructures it was decided to choose a lower temperature interval of Tc (Tc 50) for crystallization. Where Tc is DTA exothermic peak onset temperature determined as 785 °C for the coarse particle size specimen from DTA curve (Fig. 2) In order to find the suitable crystallization temperature, specimens first were subjected to the aforementioned ‘nucleation’ treatment, and then soaked for 4 h at differing crystallization temperatures in the above interval. Fig. 3 depicts the XRD patterns of some specimens heat treated according to the above procedure. Since the microstructures of glass ceramics controls the pore sizes of final products, in order to keep the pore diameters in the as low as possible range the lowest temperature showing distinct crystallization in XRD examinations, i.e. 765 °C, was chosen as the ‘crystallization’ temperature. In order to assess the actual occurrence of nucleation in the above conditions, the glass specimens after a nucleation treatment at 710 °C for differing times (6–24 h), were heated at 765 °C for 48 h and then subjected to XRD examination. Fig. 4 represents the XRD patterns of the above specimens. It can be seen that all XRD peaks for the major phases b-Ca3(PO4)2 and CaTi4(PO4)6 and the minor phase Ca(PO3)2 have continuously been intensified by prolongation of the nucleation time up to 24 h. On the other hand the SEM microstructures of two specimens nucleated for 24 and 12 h in the above conditions (Fig. 5) show that the specimen held at the nucleation temperature for 24 h developed a much finer microstructure in comparison with the specimen soaked for 12 h, indicating the formation of much greater population of nuclei in the former specimen. Moreover, in order to determine the optimum crystallization time for the specimens, they were first nucleated for 24 h at 710 °C and subsequently soaked for differing times at 765 °C. Fig. 6 shows the XRD patterns for the above specimens. Although it seems that the crystallization products continuously increased from 8 to 48 h the difference shown between 24 and 48 h is not great. On the other hand the microstructural examination by SEM revealed the occurrence of considerable growth and coarsening of crystals in the period of 24–48 h (Fig. 7). Therefore in order to keep the pore sizes in the as low as possible range, it was decided to choose 24 h as the holding time for crystal growth. In order to follow the steps of crystallization process, it was decided to carry out some short time heat treatments on the glass specimens. In these treatments the specimens after a 24 h nucleation at 710 °C, were heated at 765 °C for 15, 30, 45, 60, 90 and 120 min. Fig. 8 exhibits the XRD patterns of the heat treated specimens. It can be seen that after 45 min, considerable crystallization has occurred in the glass specimens that continuously increased up to 120 min. It is interesting to note that the type of crystallization products, namely b-Ca3(PO4)2 and CaTi4(PO4)6 as the major phases and Ca(PO3)2 as the minor phase, were not changed throughout these experiments.
Fig. 9 depicts SEM microstructures of some heat treated specimens. The crystals that started to grow after 45 min within the phase separated glass specimen (Fig. 9(c)), were spread rapidly after 60 min (Fig. 9(d)) and began to exhibit different morphologies i.e. fibrous and nearly equiaxed after 90 and 120 min (Fig. 9(e,f)). 3.3. Leaching The leaching of crystallized specimens carried out in 1 M HCl solution at differing temperatures up to 7 days were followed by measuring the bulk densities vs. time. Fig. 10 depicts the results. It can be deduced that the leaching temperature in the range of ambient temperature 60 °C has no marked effect upon the leaching process.
Fig. 6. XRD patterns for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for differing times (a) 8 h, (b) 16 h, (c) 24 h, (d) 32 h, (e) 40 h, (f) 48 h.
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Fig. 7. SEM micrographs for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for (a) 24 h and (b) 48 h.
Fig. 11 shows the microstructure of a highly porous specimen after leaching. Fig. 12 shows the XRD pattern of the porous specimen. It can be seen that CaTi4(PO4)6 and Ca(PO3)2 are the major and minor phases remained after the leaching process and there is no evidence for the presence of b-Ca3(PO4)2 phase. Table 2 summarizes some physical properties of the specimen. 4. Discussion
Fig. 8. XRD patterns for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for differing times (a) 15 min, (b) 30 min, (c) 45 min, (d) 60 min, (e) 90 min, (f) 120 min.
On the basis of the results described above it can be inferred that the most critical step in the preparation of these glass ceramics is the phase separation process. The occurrence of phase separation by an apparently spinodal decomposition mechanism (Fig. 1(c)) in the glass specimen provides the required conditions for the formation of interconnected crystalline collections of main phases,Ca3(PO4)2 and CaTi4(PO4)6 in the respective phase separated regions of glasses enriched in CaO and TiO2 respectively. Without these interconnected phases the whole process of leaching and fabrication of bodies possessing high values of fine nano-size open porosity would be impossible. On the other hand the comparison of the microstructure of a glass ceramic specimen which prior to the crystallization was subjected to a heat treatment at the phase separation temperature of 710 °C for 24 h with a specimen heated at the same temperature for 12 h (Fig. 5), indicates the existence of a much finer microstructure and hence the formation of much greater number of nuclei at the former specimen. This also resulted in much higher bulk crystallization values as proved by XRD patterns shown in Fig. 4. Therefore it can be inferred that an actual nucleation process was progressing along with the glass-in glass phase separation at 710 °C in these glasses. On the other hand the temperature interval between Tg and Td in which the occurrence of a suitable phase separation becomes possible is quite restricted. Lower temperatures would halt the already slow phase separation process and the acceleration of the phase separation process by raising the temperature may be detrimental to whole process, since this may disrupt the interconnected spinodal microstructure and convert it to a microstructure of isolated droplets distributed within a matrix. Hence it can be concluded that the relatively slow phase separation process (Fig. 1) is mainly responsible for the slow nucleation rates in these glass ceramics. The crystallization (growth) temperature is another important factor that requires careful consideration. Although DTA exo-peak temperature of 840 °C (Fig. 2), can be taken as the suitable crystallization temperature, the relatively coarse microstructures developed in
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Fig. 9. SEM micrographs for specimens nucleated at 710 °C for 24 h and subsequently heated at 765 °C for (c) 45 min, (d) 60 min, (e) 90 min, and (f) 120 min.
specimens heated at this temperature necessitated the use of lower temperatures, around the exothermic peak onset temperature (Fig. 2) e.g. 765 °C as described above. The relatively slow growth rate encountered at the chosen temperature in this experiment necessitated the use of long crystallization times, (at least 24 h). Increasing the temperature could lead to considerable coarsening of the crystals and larger pore in the final product. Therefore it seems that in the case of growth process, similar to the nucleation process, a relatively restricted condition exists in respect to the selection of suitable time/temperature for the heat treatment,
although the condition is more flexible in the case of growth. In this case a relatively limited temperature range, located between To, several degrees lower than DTA crystallization temperature of Tc = 840 °C and Tf = 765 °C can be imagined within which the variation of pore size by the variation of crystallization temperature becomes practically possible and useful. The comparison of Fig. 9(f) with Fig. 5(b), showing the crystallization of specimens after 2 and 24 h heat treatment respectively, reveals that the equiaxed particles eventually adopted plate like shapes, whereas the fibrous particles preserved their fibrous or rod-shape morphology. Also the comparison of the microstructure of porous specimens (Fig. 11) with microstructure of crystallized specimens before leaching (Fig. 5(b) or Fig. 7(a)) reveals that the rod-shape particles are those which remained in the porous specimen after the leach-
Fig. 10. Bulk density vs. time curves for leaching glass ceramic specimens at (a) ambient temperature, (b) 40 °C, and (c) 60 °C.
Fig. 11. SEM micrograph showing the porous specimen after leaching (fractured surface).
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ature, as a inherent difficulty of this fabrication method in controlling the characteristics of the resulting porous material through variation of heat treatment conditions it seems that only the compositional changes leading to higher mobility at relatively low temperatures for the ions taking part in phase separation process, with no adverse effect on the spinodal mechanism, could bring about the useful high separation and nucleation rates. The same problem is encountered in growth process. The relatively slow growth rate occurring at the chosen temperature in this experiment necessitated the use of long crystallization times, (at least 24 h). Increasing the temperature usually leads to considerable coarsening of the crystals and larger pores in the final product. This problem probably may also be solved by selecting a suitable composition, as discussed above. Fig. 12. XRD pattern of the glass ceramic specimen after leaching.
Table 2 Physical properties of porous glass ceramics. Bulk density (g/cm3)
Volume porosity (%)
Specific surface area (m2/g)
Average pore diameter (nm)
1.66 ± 0.05
41 ± 4
26 ± 3
14.3 ± 2
ing operation i.e. CaTi4(PO4)6 and the plate like particles are bCa3(PO4)2 crystals which were leached out along with some residual glass phase. Fig. 11 also reveals that the seemingly rod-shape particles of CaTi4 (PO4)6 are in fact comprised of many tiny crystals. This can be explained by the crystallization mechanism exhibited in Fig. 5(a), which is the manifestation of a spherulitic growth. In this mechanism several crystals begin to grow from an identical central nucleus in the form of radiating fibers, which branch in various directions and soon covered with numerous tiny crystallites [12]. In close observation of Fig. 11 it seems that pores seen in this figure, which are mainly in the 70–200 nm diameter range, are much larger than the measured mean pore diameter of 14.3 nm. This can be explained by the existence of numerous fine pores on the external and internal pore walls, which are observable with higher magnifications and is consistent to the findings of other investigators [6]. Although it was not possible to differentiate between the crystallization steps of different phases, from Figs. 9 and 11 it can be inferred that both major phases begin to crystallize simultaneously from different phase separated regions, rich in CaO or TiO2, and remained confined within their respective boundaries to the end of crystallization process. 4.1. Suggestion for further work On the basis of the above discussion regarding the restricted conditions for selection of nucleation and growth time and temper-
5. Conclusions It was shown that glass-in glass phase separation with a spinodal decomposition mechanism is the precursor step for the occurrence of an effective nucleation, in the glass ceramics studied herein. In this way phase separation plays a very decisive role in controlling the crystallization process and affecting the microstructure of the glass ceramics, hence determining the size and vol.% of the final porosity in the end product. The optimum phase separation–nucleation temperature and time determined as 710 °C and 24 h, in fact should be considered as an almost invariable condition for the fabrication of these type of porous materials (if composition was kept constant) with acceptable properties and nanosize porosity. The chosen crystallization (growth) condition of 765 °C and 24 h was also proved to be quite effective in producing bodies of acceptable values of porosity (41 ± 4%) and quite small (perhaps the minimum possible) mean pore diameter of 14.3 ± 2 nm. The obtained glass ceramics after leaching in 1 M HCl solution at the ambient temperature, 40 °C and 60 °C for up to 7 days did not show any considerable differences in the leaching rate or final bulk density and volume porosity values. References [1] T. Yazawa, H. Tanaka, in: K. Ishizaki (Ed.), Porous Materials, American Ceramic Society, Westerville, OH, 1993, p. 213. [2] R.A. Messing, J. Non-Cryst. Solids 26 (1977) 482. [3] I. Aso, M. Nakao, N. Yamazoe, T. Seeiyama, J. Catal. 57 (1979) 287. [4] M. Uo, M. Numata, M. Suzuki, I. Karube, A. Mkishima, J. Ceram. Soc. Jpn. 100 (1992) 430. [5] H.P. Hood, M.E. Nordberg, US patent 2, 106, 774 (1934). [6] H. Hosono, Z. Zhang, Y. Abe, J. Am. Ceram. Soc. 72 (1989) 1587. [7] H. Hosono, Y. Sakai, M. Fasano, Y. Abe, J. Am. Ceram. Soc. 73 (1990) 2536. [8] H. Hosono, Y. Abe, J. Am. Ceram. Soc. 75 (1992) 2862. [9] H. Hosono, Y. Abe, Non-Cryst. 190 (1995) 185. [10] K. Yamamoto, T. Kasuga, Y. Abe, J. Am. Ceram. Soc. 80 (1997) 822. [11] T. Kasuga, M. Nogami, Y. Abe, J. Am. Ceram. Soc. 82 (1999) 765. [12] V.K. Marghussin, A. Arjomandnia, Phys. Chem. Glasses 40 (1999) 311.