Devitrification and Phase Separation of Zinc ...

International Journal of Applied Glass Science, 5 [2] 185–192 (2014) DOI:10.1111/ijag.12049

Devitrification and Phase Separation of Zinc Borosilicate Glasses in the Presence of MgO, P2O5, and ZrO2 Mehdi Soleimanzade,* Bijan Eftekhari Yekta, and Vahak Marghussian Ceramics Division, The Department of Metallurgy & Materials Engineering, Iran University of Science & Technology, Narmak, Tehran, 1684613114, Iran

A base glass composition which undergoes phase separation was chosen in the system Na2O-ZnO-B2O3-SiO2. Spontaneous and reheat-treatment opal glasses were synthesized by addition of different amounts of oxides such as P2O5 and ZrO2. To clarify devitrification, visible spectroscopy was performed for all samples. Reheat-treated samples showed Tyndall effect in low temperatures, while they were completely opacified after soaking in 800°C for 3 h. P2O5-or ZrO2-containing samples opacified spontaneously. The mechanism of opacification in glasses containing the MgO and ZrO2 was partial crystallization, while in the mechanism of P2O5-bearing one was liquid–liquid phase separation.

Introduction The opal glasses are manufactured particularly in illumination technology, decorative purposes, glass jewelry, and tableware.1 Opals are partially crystallized or phase-separated glasses with chinaware appearance, and their opacity results from light refraction and internal scattering between either glassy or crystalline phases.2,3 The main factors determining the overall scattering coefficient and consequently affecting the opacity of a two-phase systems are the particle size, relative refractive index, and volume of the second-phase particles.4 Opacification is caused by small particles having a refractive index different from that of the matrix in which they are dispersed. On principle, it does not matter whether these particles are crystalline or not.2 *[email protected] © 2013 The American Ceramic Society and Wiley Periodicals, Inc

In this regard, there has been some interest in using sodium zinc borosilicate system because of its tendency for phase separation.5 This system has been used as opacified glass for many applications such as durable heatresistant opal glass.6,7 However, devitrification related to this important glass system in the presence of oxides with high polarization power (high charge/small radius) has not been studied sufficiently. In this respect, a glass composition was chosen to examine how its microstructure and opacity is modified by adding different oxides such as MgO, P2O5, and ZrO2. Studying the effect of these well-known opacifiers and their correlation with opacity will be a helpful and suitable approach to understand the devitrification behavior of zinc borosilicate glasses.

Experimental Table I shows the chemical compositions of the investigated glasses. Raw materials used were reagent

186

International Journal of Applied Glass Science—Soleimanzade, Eftekhari Yekta, and Marghussian Vol. 5, No. 2, 2014

Table I. Chemical Composition of Glass Samples (Parts Weight) Sample code

SiO2

B2O3

Na2O

ZnO

Al2O3

MgO

P2O5

ZrO2

B M2 P2 P4 T4 Z4

63 63 63 63 63 63

18 18 18 18 18 18

5 5 5 5 5 5

12 12 12 12 12 12

2 2 2 2 2 2

– 2 – – – –

– – 2 4 – –

– – – – – 4

grade chemicals (extra pure) H3BO3, Na2CO3, a-Al2O3, ZnO, MgCO3, H3PO4, and ZrO2. The silica was prepared through acid washing of a commercially grade silica powder. The purity of resulted silica powder was more than 99.8 wt%. Hundred gram of each glass batch was weighted, dry mixed, and melted in an electric furnace at 1500–1550°C for 1 h using alumina crucibles. The obtained glasses were melted and cast into preheated stainless steel mold, annealed at 580°C for 2 h, and then cooled naturally to room temperature. The nonspontaneously opacified series of samples were heat-treated for 1–3 h between 750°C and 850°C. In these series, the code M2-800, 3 h, for example, means that the glass M2 has been heat-treated for 3 h at 800°C. To measure the absorbance, a UV–visible spectrophotometer (PG Instrument model T80; PG Instruments Limited, Leicester, UK) was used. The samples, prepared for this purpose, were cut with approximately 2-mm thickness, polished, and lapped to form 10 9 10 9 2 mm rectangular slices were prepared for this regard. The crystallinity of opacified glasses were analyzed by X-ray diffractometry (Philips PW 1800). After polishing and etching, the samples were coated with a thin film of gold and they were subjected to scanning electron microscope examination (SEM, VEGA/TESCAN). The etching step was performed by a 2% HF solution.

Fig. 1. Immiscibility regions in the system Na2O-ZnO-B2O3-SiO2 at a SiO2:B2O3 mole ratio of 1.16:1.

Results and Discussions Fig. 2. Light transmittance of base heat-treated samples.

Base Glass Samples The base glass composition mentioned as B was selected from literature.5 Figure 1 illustrates a typical phase diagram8 in which immiscibility region of the glass for each temperature and the compositional point of glass B are depicted by tie-lines and star, respectively.

Accordingly, heat-treating of glass B at temperatures below 950°C would lead to a liquid–liquid phase separation. Figure 2 shows the transmittance spectrum of glass B after heat-treatment. Based on these results, the glass

www.ceramics.org/IJAGS

Devitrification of Zinc

187

Fig. 4. XRD pattern of sample B-800, 3 h.

Samples M2 and P2

Fig. 3. Scanning electron microscope micrograph of sample B-800, 3 h etched in 2% N HF.

B lost its transparency gradually by increasing heating temperature and soaking time. Moreover, Fig. 2 shows that the transmittance is decreased by increasing the incident light wavelength. This phenomenon is in line with Opalescence Effect. Opalescence is the name for a type of coloration (dichroism) of a highly dispersed system (crystalline or noncrystalline), the size of which is smaller than wavelength (d < k), with no or faint opacification. The material looks yellowish-red to brown when one is looking through it and purplish blue when one is looking upon it. The dichroism is caused by the prevalence in scattering of the blue part of light spectrum (short wavelengths).2 These light transmittance curves were reported for fluorotitanosilicate glasses with different heat-treatment procedures and phases of different sizes.9 According to Fig. 3, the mentioned scattering phenomenon is taking place probably due to spinodal decomposition in glass B heat-treated samples, and there is no document to show that a precipitated crystalline phase has been played a role in this regard (Fig. 4). Gradually up to 800°C, the liquid–liquid phase separation has been extended by increasing heat-treatment temperature and soaking time; subsequently, it was diminished slightly again by increasing the temperature to 850°C, as can be seen in the reduction in light transmittance in Fig. 2 (B-850, 1 h and B-850, 2 h). It seems that this temperature is above the separation dome of the present glass composition.

Adding 2 wt% MgO and/or P2O5 to the base glass led to a widespread opacification of the samples after heat-treatment at 800°C. Figures 5 and 6 represent the transmittance spectrum of heat-treated M2 and P2 glasses, respectively. X-ray diffraction (XRD) pattern of sample M2-800, 3 h (Fig. 7) illustrates that the glass is already amorphous in XRD point of view. However, based on the SEM micrographs (Fig. 8), partial crystallization has been occurred in some areas of M2. Two different crystalline morphologies—elongated and pyramidal—can be detected within the precipitated area. Figures 9 and 10 show the Energy Dispersive X-ray Spectrometry (EDS) analysis of the elongated and pyramidal crystals, respectively. Based on these results, the present authors do believe that the elongated crystals should be willemite (Zn2SiO4), and the pyramid one should be a spinel solid solution with (Zn, Mg) Al2O4 composition. The presence of willemite together with spinel has also been observed in similar glass systems.10 The phase separation in borosilicate glasses usually results in two phases: alkali-borate rich and silica rich.11 It seems that the glass–glass interfaces resulted from liquid to liquid phase separation are preferential sites for crystallization. In this case, crystallization possibly initiated and grew from the mentioned interfaces to the ward of Zn- and Mg-enriched glassy phases. The glassy phase within the middle of the crystallized area in Fig. 8a should be the silica-rich region and can be an indication for this mechanism. Therefore, it can be said that compared with base glass, the number of scattering centers and glass opacity is increased by partial crystallization of M2. This lower transmittance (2%) makes the sample M2-800, 3 h more suitable for using as an opal glass. Contrary to the base and M2 glasses, the merely annealed P2 was opal. This means that a spontaneous liquid–liquid phase separation and/or crystallization might happen during cooling of the glass melt. More-

188


Fig. 5. Light transmittance of M2 heat-treated samples.

Fig. 6. Light transmittance of P2 heat-treated samples.

Fig. 7. XRD patterns of samples M2-800, 3 h and P2-800, 3 h.

over, the light transmittances of heat-treated P2 were much lower than M2 and decreased drastically as its heat-treatment temperature and soaking time were increased (see Fig. 6). According to Fig. 7, specimen P2-800, 3 h is already amorphous in XRD point of view. Furthermore, its SEM micrograph (Fig. 11) reveals droplets, a sign for liquid–liquid phase separation mechanism through nucleation and growth.12,13 While etching, the separated phase is leached by HF.

Therefore, these droplets should be enriched in P2O5, alkaline, and alkaline earth oxides2 and are probably responsible for opacification. Samples P4 and Z4 Samples P4 and Z4 were opacified spontaneously during melt casting. As seen in Fig. 12, the light transmittance of both samples is less than 0.5%.


(a)


189

(b)

Fig. 8. Scanning electron microscope images of sample M2-800, 3 h (a) magnification 93000, (b) magnification 920,000; “W” indicate the willemite phase and “S” indicates spinel phase.

Fig. 9. EDS analysis of the elongated crystal (W).

Fig. 11. Scanning electron microscope micrograph of P2-800, 3 h etched in 2% N HF.

Fig. 10. EDS analysis of the pyramidal crystal (S).

Economically, spontaneously opacifiable glasses are preferred over glasses requiring a subsequent heattreatment.7 Adding 4 wt% P2O5 and ZrO2 to the base glass composition increased the rate of opacification so that as-received glasses lost nearly all of their transparency without any further heat-treatments. According to SEM micrograph of etched sample P4 (Fig. 13), by increasing P2O5 amount, the immiscibility mechanism of the glass has been changed from the nucleation and

190


Fig. 12. Light transmittance of samples P4 and Z4.

Fig. 14. XRD patterns of P4 and Z4.

Fig. 13. Scanning electron microscope micrograph of P4 sample etched in 2% N HF.

growth in glass P2 to the spinodal decomposition in P4. Moreover, similar to P2, as-received glass P4 was completely amorphous in point of XRD criterion (Fig. 14). It is said that the system P2O5-RO (where RO = BaO, SrO, CaO, MgO, ZnO, CdO, PbO) becomes glassy up to 56–65 mol% of RO,1 and large amounts of ZnO can be dissolved in a phosphorous glass matrix phase without any crystallization. Meanwhile, P5+ in P2O5 is discordant with Si4+ in SiO2 because of one extra positive charge. This discordancy

leads to phase separation of phosphate groups from silicate matrix, and the separation is intensified while the amount of the P2O5 increases in the glass.14 Therefore, here one of the phase-separated areas should be possibly enriched with phosphor and zinc oxides which has a less refractive index than the silicate phase.15 This specification and occurrence of extensive separation should be responsible for opacification. XRD patterns of as-received glass Z4 (Fig. 14) showed that it has been crystallized to tetragonal zirconia, willemite (Zn2SiO4), and gahnite (ZnAl2O4) during melt casting. Figure 15 illustrates the microstructure of this crystallized glass. According to EDS analysis of Fig. 16, the prismatic crystals (illustrated by arrows in Fig. 15b) are gahnite, the shiny particles in Fig. 15a are zirconia, and the spherulitic crystals are willemite, one of



(a)

191

(b)

Fig. 15. Scanning electron microscope images of Z4, (a) magnification 915,000, (b) magnification 930,000.

parts weight P2O5 caused a liquid–liquid phase separation mechanism through nucleation and growth; increasing it to four parts weight led to a different mechanism of separation, that is, spinodal. On the other hand, ZrO2 acted as a nucleating agent for crystallization of gahnite and willemite in the zinc borosilicate glass system. Besides, ZrO2 caused a rapid crystallization of the glass system so that it became opal spontaneously by addition of four parts weight ZrO2. References Fig. 16. EDS analysis of prismatic crystals marked by arrows.

which is located in the middle of magnified section.16,17 The presence of willemite and gahnite phases is because of the presence of ZrO2. It seems that ZrO2 developed as the primary crystal phase acts as a nucleating agent for crystallization of willemite and gahnite. According to Fig. 15, the crystallites of the latter phases are about 1 lm in size. Apparently, these relatively large crystals are responsible for high degree of opalescence in glass Z4.

Conclusions The effects of MgO, P2O5, and ZrO2 on a zinc borosilicate glass were investigated. MgO enhanced the devitrification affinity of glass and led to partial crystallization of willemite and spinel. An addition of two

1. G. B. Rothenberg, Glass Technology, Recent Developments, 211–212, Noyes Data Corp., Park Ridge, NJ, 1976. 2. W. Vogel, N. J. Kreidl, and E. Lense, Chemistry of Glass, American Ceramic Society, Columbus, OH, 1985, p. 202. 3. D. C. Boyd and J. F. MacDowell, American Ceramic Society, Glass Division, Opal Glass in Commercial Glasses, 141–150, American Ceramic Society, Columbus, OH, 1986. 4. W. D. kingery, D. R. Uhlmann, and H. K. Bowen, Introduction to Ceramics, 2nd edition, John Wiley and Sons, NewYork, NY, 1976, p. 666. 5. P. Taylor and D. G. Owen, “Liquid Immiscibility in the System Na2OZnO-B2O3-SiO2,” J. Am. Ceram. Soc., 64 360–367 (1981). 6. J. E. Flannery, J. L. Stempin, and D. R. Wexell, “Fluorophosphate Opal Glasses”, U.S. Patent 3661601, 1981. 7. R. M. Wiker, “Zinc Borosilicate Opal Glasses,” U.S. Patent 4376170, 1983. 8. P. Taylor, “A Review of Phase Separation in Borosilicate Glasses, With Reference to Nuclear Fuel Waste Immobilization, Whiteshell Nuclear Research Establishment, Atomic Energy of Canada AECL-10173,” (1990) 22. 9. W. Vogel, N. J. Kreidl, and E. Lense, Chemistry of Glass, American Ceramic Society, Columbus, OH, 1985, p. 196. 10. M. C. Bekir Karasu and S. Turan, “The Development and Characterisation of Zinc Crystal Glazes used for Amakusa-Like Soft Porcelains,” J. Eur. Ceram. Soc., 20 2229 (2000). 11. J. E. Shelby, Introduction to Glass Science and Technology, 70–71, Royal Society of Chemistry, Cambridge, UK, 2005. 12. R. H. Doremus, Glass Science, 60–66, John Wiley & sons, Inc, Rensselaer Plytechnic Institute, New York, 1994. 13. O. V. Mazurin and E. Porai-Koshits, Phase Separation in Glass, 165–167, Amsterdam and New York, North-Holland, 1984.

192


14. P. W. McMillan, Glass-Ceramics, 76–77, Academic Press, London; New York, NY, 1979. 15. G. Walter, U. Hoppe, J. Vogel, G. Carl, and P. Hartmann, “The Structure of Zinc Polyphosphate Glass Studied by Diffraction Methods and 31P NMR,” J. Non-Cryst. Solids, 333 252–262 (2004).

16. W. H€oland and G. H. Beall, Glass-Ceramic Technology, American Ceramic Society, Westerville, OH, 2002, p. 115. 17. B. E. Yekta, P. Alizadeh, and L. Rezazadeh, “Floor Tile Glass-Ceramic Glaze for Improvement of Glaze Surface Properties,” J. Eur. Ceram. Soc., 26 3809–3812 (2006).