Crystallization and sintering behavior of phlogopite–soda lime composite

LETTER TO THE EDITOR Journal of Non-Crystalline Solids 357 (2011) 3385–3391

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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l

Letter to the Editor

Crystallization and sintering behavior of phlogopite–soda lime composite A. Faeghi-Nia ⁎ Ceramic Division, Materials and Energy Research Centre, P.O. Box: 14155-4777, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 9 March 2011 Received in revised form 24 May 2011 Available online 27 June 2011 Keywords: Glasses; Mechanical properties; Thermal analysis; Sintering

a b s t r a c t Sintering of (Soda-lime) glass, by means of 20–40 wt.% phlogopite, has been studied. According to TG and FT-IR, water loss of composite s by 8 wt.%, up to 1000 °C, caused the bloating and reduced the relative density. Dissolution of phlogopite with Diopside crystallization was detected up to 900 °C. Hardness of composites was increased by phlogopite content from 320 to 694 HV and flexural strength was obtained in the range of 30–80 MPa. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Powder technology is the processing route most commonly used to fabricate ceramic products. Also, most composite materials with glass matrix, containing dispersion reinforcement are obtained by the powder sintering route. The presence of heterogeneities in the structure of a green compact can have significant effects on the sintering behavior and consequently on the final properties of the ceramic products [1–5]. Glass matrices exhibit a much higher sinterability compared to crystalline matrices due to the ability of the viscous glass phase to relax the shear stresses developed as a consequence of differential sintering rates [6,7]. The sintering of glass has been widely investigated providing results about densification and flow during sintering [1,6,8]. The glass–ceramic composites, containing glass as a matrix have been studied in Alumina–glass [3] Zirconia–glass [7,9] Cordierite–glass [10]. Non-reactive liquid-phase sintering of these systems by means of classic theories was reported. But there is no systematic studied about the phlogopite–glass composites. Sintered phlogopite–Glass composites as porous and dense insulate material were introduced in 1980 [11–14]. These composites were made from the local area's phlogopite and window glass waste. First the thermal insulation properties of this composite were distinguished. It has been reported that the composite by 20 wt.% phlogopite and with 300 μm grain size, shows good compressive strength and low water absorption, also the bloating and abnormal expansion of these systems, have been assumed to the removing water from phlogopite, but the sintering kinetic and crystallization behavior have not been studied.

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In 2008, Weijing [15] has shown the machinability of these systems and examined the microstructure and crystalline phases in a single composite with 35 wt.% phlogopite–65 wt.% glass. The studied particle size was nearly in the range of 120–149 μm. He has detected the crystalline Amphibol phase at 1000 °C in this system. No evidence of sintering kinetic, bloating, poor sintering in composites with more or less than 35 wt.% phlogopite has been reported in this work [16]. Since the insulate–machinable system produced with a significant amount of waste glass, is a considerable economic material, it sounds that sintering and crystallization mechanism as well as bloating of these systems should be studied. In the present work the particle sizes of phlogopite and glass were 60 μm, and the effects of temperature, time, phlogopite content, as well as bloating of composite systems were studied systematically.

2. Experimental 2.1. Materials preparation Glass powders used for present investigation were prepared from recycled colorless soda-lime glass cullet, supplied by a local glass manufacture, Gazvin Glass Ltd. The phlogopite type phlogopite is processed in Varom gia. Oromiea was from a high purity ore under the trade name known as phlogopite. The chemical analysis of glass and phlogopite is represented in Table 1. The glass cullet was, first pulverized into fine grains (in the range of 0.2–0.5 mm in diameter) by a mechanical pulverize. The pulverized glass grains were then ground into fine powders by a wet ball millgrinding process for a period of about 70 h, using corundum grinding media. The glass and phlogopite powders used for samples preparation were mostly in range of the 25 and 62 μm respectively. Typical particle size distribution of prepared powders which has been measured by

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A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

Table 1 The chemical analysis of glass and mica.

Glass Mica

SiO2

K2O

Na2O

MgO

CaO

Al2O3

Fe2O3

72.9 37.3

– 10.1

14.4 –

3.2 24.6

5.9 2.7

2.7 15.3

– 3.19

Fritsch Particle Size Analysis, (analysette 22) are shown in Fig. 1a, b and also Fig. 2 represents the XRD pattern of phlogopite. Weight ratios of phlogopite/ glass composition, were 20/80, 30/70 and 40/60 the three specimens are denoted by, C28, C37, C46, respectively. For most experimental evaluation purposes, a 50-gram specimen was prepared from appropriate proportions of the recycled glass powders and phlogopite powders. Mixing of powders was carried out using water as a wet media and an ordinary ball mill with alumina balls for 2 h. The obtained mixture was dried at 120 °C. Mixed powders were then compressed into a circular disc measuring about 50 mm in diameter and 10 mm in thickness employing a compaction pressure of about 70 MPa. Compressed powder compact was then placed on a refractory brick and sintered in an electric heating furnace under atmospheric conditions. Temperatures within the range of 850 °C to 1050 °C and soaking periods varying from 10 min to 120 min were used. Temperature of the furnace was raised to the selected level steadily with a heating rate of approximately 5 °C/min. Cooling rate of furnace was 10 °C/min. To determine the physical properties, such as bulk density, apparent porosity, water absorption, and volume change, the sintered specimens, were characterized along with the procedure described in the ASTM-C20 methods.

Fig. 2. Represents the XRD pattern of mica.

Flexural strength tests were carried out on an Instron machine at room temperature by using four-point bending with a 20 mm span between the inner rods and 40 mm span between the outer rods. 2.2. Characterization A Vickers Microhardness tester with a diamond pyramid (MVK-H21 Microhardness) was used to measure the micro hardness of composite's surface by applying a load of 50 g for 30 s. Thermal expansion coefficients of sintered composites were measured by a dilatometer (Netzch, E402). Mixed powders were subjected to thermal analysis using simultaneous TG/DTA (Model: 320). Crystalline phase identification was performed on powder made from ground sintered pellets, using X-ray diffraction (Philips Power Diffractometer 1710) with Ni-filtered Cu-Kα radiation and the relevant JCPDS cards (Joint Committee on Powder Diffraction, 1972). Scanning electron microscopy (SEM: model JEOL JXA-840) was used in order to observe the microstructure and EDX analysis. Samples were mounted onto the sample holder, coated with gold, and then studied with SEM. Fourier transform infrared spectroscopy (FT-IR-Bruker-Vector 33) was used for studying the structural water. 3. Results Thermal analysis of phlogopite, glass and C37 is shown in Fig. 3. 6 wt.% weight losses along with endothermic peak, at 650 °C, associated to the loss of interlayer water, were presented in phlogopite. Moreover, an exothermic peak at 850 °C, (based on previous works) [17], correlated to the structural change of phlogopite was detected. Thermal events of phlogopite, and two other small exothermic peaks, at 760 °C and 950 °C 101

0

100

-1

99

-2

98

-3

97

-4

96

-5

95

-6

94

-7

93

-8

tg phlogopite tg glass glass

92 0 Fig. 1. A. particle size distribution of mica powders. B. particle size distribution of glass powders.

200

400

600

800

phlogopite

-9 1000

Fig. 3. Thermal analysis corresponding to mica, glass and C37.

LETTER TO THE EDITOR A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

110% 100%

Relative density

were distinguished in C37. Also continuous 4% weight loss of glass up to 700 °C was significant. To identify the volatile mat, raw glass and heat treated glass at 850 °C were analyzed by FT-IR; (Fig. 4). The intensity of principle peak at 3440/cm, related to the molecular water, was reduced at 850 °C (Fig. 4). Source of this structural water, can be estimated from raw materials, used in the glass melting, or to the wet milling, which was remained in the glass structure. Relative density vs. temperature of composites are plotted in Fig. 5, the shrinkage of composites starts at 850 °C, and final relative density of C37 is 97% at 950 °C, whereas, bloating of C28 at 950 °C suppresses complete densification. The weight loss of composites is presented in Fig. 6. To study the time effect on sintering behavior, the relative densities of composites vs. log t, at different temperatures was measured and represent in Fig. 7. According to Fig. 7(a and b), C28 and C37 densities , at 850 °C and 900 °C, are time dependent, this dependence is reduced, by rising temperature to 1000 °C. Fig. 8 shows the change of (radial and axial) shrinkages of C28 and C37 with temperature; all the values represent the average from at least three samples. It was observed that for a given sintered compact the axial shrinkage (Δl/lo) was always higher than the radial shrinkage (Δr/ro) and that the increase in phlogopite content favored the anisotropic shrinkage behavior as illustrated in Fig. 8. To classify any crystallization, and possible reactions among glass and phlogopite, glass and C37 were heat treated and analyzed by XRD at first exothermal peak (760 °C). (Fig. 9). Carnegieite (NaAlSiO4) has been distinguished as the sole crystalline phase in glass, , this phase has been reported by others previously [22], Whereas Diopside, along with phlogopite was developed by heat treating of C37 at 760 °C. Fig. 10 shows the phase evaluation of the composites at their optimum sintering temperature. Obviously, sintering up to 950 °C leads to the crystallization of Diopside in composites. Since this temperature is near to the second exothermic peak (in Fig. 3), the

3387

c37

c46

90% 80%

c28

70% 60%

C28 C37

50%

C46

40% 800

850

900

950

1000

1050

1100

temperature(c) Fig. 5. Relative density vs. temperature of composites (comparing to other works).

mentioned Diopside phase, is beginning to form, at this temperature, and characteristic peak of phlogopite disappears gradually up to 900 °C in C37. To study the effect of trapped water on Tg of remained glass and CTE (thermal expansion confession) of composites, the dilatometric analysis was performed, on pre-sintered composites at 900 °C. The results are presented in Fig. 11 as well as Table 2. Also according to Fig. 11 the glass transition of C28 (520 °C), is lower than the Tg values of glass which was obtained in Fig. 3. Since hydroxyl ion and alkaline earth oxides, reduces the glass transition of glass, [26] this evidence, may approve that the remaining glass of C28, has more hydroxyl ions resulted from phlogopite. Microhardness, bending and compressive strengths of sintered composites were measured and compared. According to the results of Table 2, apparently, by increasing the phlogopite content, the microhardness is mounted up to 694 Kg f/mm 2. However obtained values of

Fig. 4. FT-IR analysis of raw glass and heat treated at 850 °C.

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A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

Fig. 6. Weight losses of composites, at different temperatures.

Fig. 8. Radial and axial shrinkage of C28 and C37.

bending strength are lower than conventional Diopside glass-ceramics [25], related to the porosities resulted from degassing. The obvious differences between microstructures of C46 and C28 heat treated at 900 °C are presented in Fig. 12(a) and (b) respectively. Although, both composites have Diopside phase, but C46, has more open porosities. In addition EDX analyses of remained glasses in C28 and C46, detect more potassium content in C46 as expected and cause more degassing (Fig. 12c). A noticeable amount of high density, Diopside crystals in the case of C46 are detected as well in Fig. 13. This phase can introduce some porosity into the system, during densification and thus may prevent complete densification, too.

A

relative density

c28

80

c37

75

c46

850C

70 65 60 55

4. Discussion

50 1.5

1

2

2.5

log t

relative density

B 76

c28

74

c37

900C

72 70 68 66 64 62 0

1

2

3

log t

relative density

C

c28

104 100 96 92 88 84 80 76 72 68 64 60

c37 c46

1000C

1

1.2

1.4

1.6

1.8

2

log t Fig. 7. (A,B,C), densities of C28 and C37 at 850 °C , 900 °C, 1000 °C.

2.2

It has been found that [18] the more MgO/CaO ratio in soda-lime composition caused the more water diffused in to the structure. In our system, possibly wet and long time milling, led to the 4 wt.% diffusion of molecular water in glass. Water removal results in bloating and consequently high amount of porosities. So the obtained sintering temperatures are higher than other report [15]. Also gradual weight loss of composite corresponds to phlogopite, and glass, will be enhanced by increasing the phlogopite content. It is well known that [19,20] potassium oxide decreases the surface tension of glasses, so by dissolution of phlogopite, potassium ion diffuses in to the glass structure of C46 and C37, causes low surface tension, since the high surface tension is necessary for sintering [21], open porosity between the glass particles are not sealed in C37 and C46, allowing more time for gas to escape without forming bubbles. Hence by increasing the phlogopite content, due to reduced surface tension the escape of gas will be facilitated [21]. Lower surface tension may have slowed, the sintering of glass particles, by reducing the solid–vapor interfacial energy, driving force for reduction of the powder surface area [20]. Furthermore the low density of sintered C46 (comparing to C37) can be explained by the high amount of crystallization that increases the viscosity and produces porosities [15]. Rahman [1] has found that the lenticular shape of the pores in the soda-lime glass compacts will tend to increase the axial shrinkage, in addition it is implicated that, the particles rearrangement of phlogopite assisted the axial densification of composites. According to XRD results in Fig. 10, either phlogopite dissolved partially, changes the ions concentration of residual glass, or like a nucleating agent, encourages the Diopside formation in composites systems. The amphibole crystals which were reported in previous work [15] are not detected in our system. It has been shown [23] that in phlogopite(mica) glass-ceramic systems, if the mol% of CaO/MgO is

LETTER TO THE EDITOR A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

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Fig. 9. XRD analysis of glass and composite heat treated at 760 °C cargeneite, d: diopside, p: phlogopite.

Fig. 10. Phase evaluation of the composites at their optimum sintering temperature. C: p: phlogopite, d: diopside.

up to 0.25, amphibole crystallization at high temperature, will be promoted, this factor is 0.44 in Zhang’s composite [15] and 0.16 to 0.27 in our composite. Considering the phlogopite dissolution in sodalime matrix, and the lack of amphiboles formation in our system, it would be assumed that CaO has major role in amphibole configuration within soda lime–phlogopite composites. The CTE of composites is increased by phlogopite content, other investigators also reported that [24] in the Diopside glass-ceramics, increasing alkaline earth oxide, will raise crystallization rate, and will causea high CTE remained glass to form [25], this, may explain, the high value of composite's CTE as a function of alkaline earth oxide content, resulted from phlogopite dissolution. It is well known that there is a direct correlation between the microhardness, and K + concentration in soda-lime glass [20]. It can be

supposed, in the present systems, as K2O did not enter in to the crystalline phase and remained in the residual glass matrix, glass hardness is increased.

5. Conclusion In this work the soda-lime glass as a matrix and phlogopite mineral as a second phase was sintered. Obtaining the dense glass matrix composites, by sintering, requests fine glass powder, and consequently wet milling. We used wet milling to produce b60micron glass powder so water induced into the glass structure, and these unavoidable hydroxyl ions, shows bloating at different sintering temperatures. It has shown that low surface tension of glass, resulted from phlogopite dissolution up to 900 °C , may decrease the bloating and delay the sintering process. Also, phlogopite changes the chemical composition of glass and encourages the Diopside formation. Diopside, glass-ceramic with the high CTE was formed. Hardness of composites were increased by phlogopite content from 320 to 694 HVflexural and compressive strengths were obtained in the range of 30–80 MPa and 90–105 MPa respectively.

Table 2 Microhardness (Kgf/mm2), Bending, comp. strength (MPa) and CTE (thermal expansion confession) of composites.

Fig. 11. The dilatometery analysis of composites pre-sintered at 900 °C.

Samples code

C28

C37

C46

Bending strength (MPa) Comp. strength (MPa) Microhardness (Kg f/mm2) CTE (thermal expansion coefficient) × 10−6/°C

35 ± 5 90 ± 2 320 ± 52 9.5 ± 0.01

84 ± 3 92 ± 9 597 ± 76 9.7 ± 0.1

72 ± 2 105 ± 5 694 ± 82 10.21 ± 0.05

3390

LETTER TO THE EDITOR A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

Fig. 12. (A) and (B) microstructures of C46 and C28 heat treated at 900 °C. (C)EDAX of matrix in C28 (D) in C46.

LETTER TO THE EDITOR A. Faeghi-Nia / Journal of Non-Crystalline Solids 357 (2011) 3385–3391

Fig. 13. Diopside crystals in the case of C46 are detected.

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