MATERIALS CHEMSHTf~W4~D Materials Chemistry and Physics 42 (1995) 5661
Suppression of crystallization during sintering of lead borosilicate glass powders Jau-Ho Jean, Chia-Ruey Chang, Tong-Hua Kuan, C.H. Lin Department
of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, ROC Received
1995; revised 8 March 1995; accepted 8 March 1995
Abstract Cristobalite is formed when an initially amorphous lead borosilicate glass is sintered at temperatures ranging from 700 to 1000 “C. X-ray diffraction results show that with a sufficient amount of gallium oxide present, however, the precipitation of cristobalite is completely prevented. The amount of gallium oxide required to inhibit cristobalite formation decreases with increasing sintering temperature. The above results are further evidenced by the linear thermal expansion measurement, in which the thermal expansion of the cristobalite-free sample is much smaller than that with cristobalite. The resulting cristobalite-free glass composite has a thermal expansion coefficient of 3.5 X lo-’ K- ’ in the temperature range of 20-200 “C, and a dielectric constant of 5.1 at 1 MHz. Keyword.~; Crystallization;
Lead borosilicate glass [ 1] is of interest for application in preparing low-temperature, low-dielectric ceramic substrates because it has desirable properties [2,3] such as a low dielectric constant of 4.6 at 1 MHz, and a thermal expansion coefficient of 3.4 X lop6 K- ’ in the temperature range of 20200 “C. Moreover, it also has a softening point at 700-7.50 “C which promotes the densification of glass + ceramics at temperatures in the range of 800-900 “C, close to that needed for co-firing with high electrical conductivity metals such as Cu, Ag and Ag-Pd. The major problem with this glass, however, is the formation of cristobalite during sintering at 700900 “C [ 41. The precipitation of cristobalite in the initially amorphous lead borosilicate glass matrix is quite disadvantageous because its large thermal expansion coefficient (TCE) and volume change associated with a martensitic transformation from the high cristobalite to the low cristobalite phase at about 200-270 “C  reduce the thermal shock resistance and mechanical strength of the glass composite. Prevention of cristobalite formation may make this glass useful in forming low-temperature, low-dielectric glass + ceramic substrates. It is therefore the objective of this investigation to prevent lead borosilicate glass from forming cristobalite during firing by adding a devitrification inhibitor. Several cristobalite growth inhibitors have been identified [ 6,7], gallium oxide 0254-0584/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved .%SDIO254-0584(95)01555-g
being one of them. In this paper we describe the effect of gallium oxide on the precipitation of cristobalite in lead borosilicate glass.
The as-received lead borosilicate glass powder (SEM COM, Cleveland, OH) was used in this study. The glass powder had a median size of 7 pm and contained 70-75 wt.% SiOZ, 15-20 wt.% B203, 5-6 wt.% PbO, 2-4 wt.% Na,O and l-2 wt.% Al,O,. The gallium oxide powder (Alfa Products, Danvers, MA) with a median size of 2 pm was used. X-ray diffraction (XRD) analysis was used to determine the crystallinity of powders and sintered compacts. It was found that the as-received lead borosilicate glass powder was amorphous, and the gallium oxide powder exhibited pphase. The powders were mixed with 5 wt.% polyethylene glycol binder in I-propanol. The suspension was de-agglomerated by a high energy ultrasonic horn, and mixing was continued using a Turbula mixer (MaschinenfabrikBasel, Switzerland) for 2 h. The powder mixture was dried, ground and uniaxially pressed at about 90 MPa to make pellets 1.3 cm diameter and 0.3 cm high. Pure glass powder was also processed similarly as a control. Samples were sintered isothermally in air at different temperatures for various periods of time. The aver-
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age linear thermal expansion coefficient (TCE) from room temperature to 200 “C was measured in air at a heating rate by using a thermal mechanical analyzer of 5 “C mini (TMA) . Dielectric constant and dissipation factor were measured by a Hewlett-Packard (HP) 4192 a.c. impedance meter at 1 MHz. Microstructures of the sintered samples were examined with a scanning electron microscope (SEM) , and elemental distributions were determined using wavelength dispersion spectroscopy in a microprobe.
3. Results and discussion 3. I. Chemical reaction between gallium oxide and lead borosilicate glass It has been previously demonstrated  that to prevent cristobalite from forming in single or binary glass systems during sintering, several issues must first be addressed. (1) Does the strong coupling between Ga3+ from gallium oxide and Na+ from lead borosilicate glass exist? (2) Is the above coupling reaction rapid enough, compared with cristobalite formation, to prevent cristobalite from forming? The samples used for investigating the chemical reaction between gallium oxide and lead borosilicate glass were prepared by placing a piece of lead borosilicate glass, approximately 3 x 3 x 0.2 mm, between two gallium oxide substrates and fired at several temperatures for various periods of time in air. Note that the gallium oxide substrate was prepared by firing gallium oxide powder compact at 1500 “C for 8 h in air, and a relative sintered density greater than 98% was obtained. Moreover, the fired gallium oxide substrate was polished using 0.3 pm alumina powder for 15 min before forming a diffusion couple with lead borosilicate glass. Figs. 1 (a)-(e) show a typical SEM micrograph along with elemental mappings of Ga, Pb, Na and Si, respectively. An interdiffusion layer, which can be identified based upon image contrast in Fig. 1 (a), is built up at the gallium oxide/ lead borosilicate glass interface and moves toward lead borosilicate glass with time. Moreover, no cristobalite is formed in the reaction layer, in contrast to the crystalline phase that is formed away from the interface of gallium oxide/lead borosilicate glass. Figs. 1 (b)-(e) clearly show that the Pb*+ and Na+ from lead borosilicate glass diffuse to the gallium oxide interface and Ga3+ from gallium oxide dissolves into lead borosilicate glass to form an interdiffusion layer that is located adjacent to gallium oxide. The above phenomenon is similar to our previous studies on the reaction kinetics between gallium oxide and Pyrex borosilicate glass systems [ 91. For comparison, the typical microstructure for the pure lead borosilicate glass sample fired at 900 “C for 8 h is shown in Fig. 2. Clearly, the precipitated cristobalite crystals can be readily identified in Fig. 2(a). Figs. 2(b)-(d) are elemental mappings of Pb, Na and Si, respectively, showing that the initially amorphous, homogeneous lead borosilicate glass has separated into two distinct phases, sodium and lead-rich
Fig. 1. (a) SEM micrograph for the diffusion couple between gallium oxide and lead borosilicate glass fired at 900 “C for 8 h, and elemental mapping of (b) Ga, (c) Pb, (d) Naand (e) Si.
amorphous and sodium-deficient crystalline (cristobalite) phases. A comparison of the results shown in Figs. 1 and 2 clearly indicates that the strong coupling reaction between Ga3’ from gallium oxide and Na+ and Pb2+ from lead borosilicate glass is directly associated with the inhibition of cristobalite in lead borosilicate glass.
J.-H. Jean et al. /Materials
Chemistry and Physics 42 (1995) 56-61
gallium oxide and amorphous phase, indicating that the amount of gallium oxide added is sufficient to inhibit cristobalite from forming in lead borosilicate glass. A microstructural analysis was then conducted on the polished surface of the gallium oxide-doped samples. Fig. 3(a) shows that the cristobalite crystals observed in Fig. 2(a) disappear completely. Moreover, the results shown in Figs. 3(b)-(e) also
Fig. 2. (a) SEM micrograph for pure lead borosilicate glass fired at 900 “C for 8 h , and elemental mapping of (b) Pb, (c) Na and (d) Si.
To verify further the role of gallium oxide in prevl enting tryst: dlization from lead borosilicate glass, the gallium oxide substl rate is replaced by gallium oxide powder. For the initial exper ,iment, we had arbitrarily selected a composition of 90 vol.% I lead borosilicate glass and 10 vol.% gallium oxi de for micrc bstructural study. The disks were fired at 900 “C fcor 8 h. XRD analysis shows no cristobalite but the presence of p-
Fig. 3. (a) SEM micrograph for the sample with 90 vol.% lead borosilicate glass-10 vol.% gallium oxide fired at 900 “C for 8 h, and elemental mapping of(b) Ga, (c) Pb, (d) Naand (e) Si.
J.-H. Jean et al. /Materials Chemistryand Physics42 (1995) 56-61
indicate that the Ga3 + from gallium oxide and Na+ and Pb2+ from lead borosilicate glass are strongly coupled and exhibit the same geometric configuration in the microstructure. The above results, similar to those observed in the Pyrex borosilicate glass system , show that Na+ and Pb*+ in lead borosilicate glass have a greater affinity for Ga3+ than for lead borosilicate glass, thus reducing the tendency of phase separation and devitrification in lead borosilicate glass. 3.2. XRD analysis -
lium oxide content may not be rapid enough, compared to the rate of cristobalite formation, to have all Na+ and Pb*+ in the lead borosilicate glass segregated at the interface of gal-
effect of gallium oxide concentration
Having thus confirmed the required coupling reaction between Ga3+ from gallium oxide and Na+ and Pb*+ from lead borosilicate glass, it is now instructive to investigate the effect of gallium oxide concentration on the devitrification kinetics and mechanism of lead borosilicate glass. Samples used for XRD analysis were prepared by adding various amounts of gallium oxide to lead borosilicate glass, processed as before, and fired at temperatures from 700 to 1100 “C for 8 h. Fig. 4 summarizes the XRD results for the samples with c-20 vol.% gallium oxide powder. It is found that the reaction products formed between gallium oxide powder and lead borosilicate glass are closely related to gallium oxide content and sintering temperature. Moreover, the results shown in Fig. 4 exhibit four major phase zones: cristobalite, amorphous, cristobalite + gallium oxide and gallium oxide + amorphous. Details of the formation of each of these phases is now discussed. Cristobalite This phase is marked by A in Fig. 4. In addition to the presence of cristobalite in gallium oxide-free samples at temperatures from 700 to 900 “C (Fig. 5), cristobalite is also the only crystalline phase detected by XRD for the samples with l-2.5 vol.% gallium oxide (Fig. 6)) indicating an insufficient amount of gallium oxide to inhibit cristobalite formation. Cristobalite + gallium oxide In this composition-temperature area (marked by A in Fig. 4)) cristobalite and unreacted gallium oxide are detected (Fig. 7). Moreover, the highest temperature at which cristobalite is observed decreases as the gallium oxide content increases, from 700 “C for above 20 vol.% to 900 “C for 5 vol.%. Since no cristobalite is observed in the interdiffusion layer between gallium oxide and lead borosilicate glass (Fig. 1) , the cristobalite observed in the presence of gallium oxide in Fig. 4 must be attributed to the incompletion or sluggishness of this reaction, compared to cristobalite formation. Support for this argument is provided by the experimental observation that the XRD peak intensity of cristobalite ( 100) decreases with increasing gallium oxide content at a given temperature, e.g., from 10 779 cps for 1 vol.% gallium oxide to 140 cps for 20 vol.% gallium oxide at 700 “C. The implication of the above result is that the chemical reaction between Ga3 + from gallium oxide and Naf and Pb*+ from lead borosilicate glass for the systems with insufficient gal-
Fig. 4. XRD results for the samples with O-20 vol.% gallium oxide fired at 70&l 100°C for 8 h
1100 0 7
27 31 28 deg.1
5. XRD results for pure lead borosilicate
glass fired at 700-l 100 “C for
-__ x Crlstobollte 175 Temp ('Cl 150-
6. XRD results for the sample with 2.5 vol.% gallium oxide fired at 700- 1100 “C for 8 h
J.-H. Jean et al. /Materials Chemistry and Physics 42 (1995) 5661
x Crlstobolrte o Ga,Oa
that the formation of a Ga3 + and Na + and Pb’ + -rich reaction layer around the gallium oxide particle is kinetically preferred to cristobalite formation. 3.3. Thermal expansion and dielectric measurement
Fig. 7. XRD results for the sample with 10 vol.% gallium
oxide firedat 700-
1100”CforSh. lium oxide/lead borosilicate glass to prevent phase separation or devitrification in the lead borosilicate glass. Amorphous The phase is marked by 0 in Fig. 4, appearing at temperatures greater than 1000 “C for the samples with gallium oxide content less than 5 vol.%. For gallium oxide-free samples (Fig. 5)) the absence of cristobalite formation can be explained by the fact that the lead borosilicate glass has a liquidus temperature of 965 “C [ 11. For the compositions with l-5 vol.% gallium oxide (Fig. 6), the gallium oxide powder clearly dissolves in lead borosilicate glass at a temperature above its liquidus temperature, forming a new glass. Gallium oxide + amorphous In this composition-temperature zone (marked by 0 in Fig. 4)) only unreacted gallium oxide (Fig. 7) is detected, indicating that the amount of gallium oxide added exceeds the requirement for inhibiting the growth of cristobalite from the lead borosilicate glass under the experimental conditions investigated. It is also noted that the required gallium oxide content to inhibit cristobalite formation completely decreases with increasing temperature, from above 20 vol.% at 700 “C to 2.5-5 vol.% at 900 “C, indicating an increased reaction kinetics between gallium oxide and lead borosilicate glass at an elevated temperature. The above results can be interpreted from a kinetic viewpoint, as demonstrated previously in the gallium oxide/Pyrex borosilicate glass [ 91 and alumina/Pyrex borosilicate glass [ 61. There, we demonstrated that the condition for preventing cristobalite formation was to make the reaction time between gallium oxide and lead borosilicate glass shorter than that of incubation for cristobalite formation, otherwise devitrification takes place. In view of the similarities in chemical composition and devitrification behavior between the present system and the gallium oxide/Pyrex borosilicate glass [ 91, the inhibition of cristobalite formation must be due to the fact
The above results can be further verified by thermal expansion measurement. Since cristobalite has a high value of TCE (50 X lo-” K- ’ ) [ 1 ] compared with that of lead borosilicate glass (3.4 X 1O-6 K- ‘) [ 11, the precipitation of cristobalite will increase the thermal expansion of glass composite with increasing amount of precipitated cristobalite. Fig. 8 shows the results of thermal expansion measurement at temperatures from room temperature to 500 “C for the samples with 0 and 10 vol.% gallium oxide fired at 900 “C for 4 h. It is clearly noted that both the magnitude of relative expansion and the slope of the curves at a given temperature decrease with increasing gallium oxide content. For comparison, the typical thermal expansion curve of pure cristobalite as a function of temperature is also included in the inset of Fig. 8. It is found that the dramatic change in slope observed at 200 “C for pure cristobalite, corresponding to a displacive transformation from low cristobalite to high cristobalite phase [ 51, is also observed in the gallium oxide-free sample, again indicating the presence of cristobalite which confirms the XRD results in Fig. 4. No such phenomenon, however, is observed when the sample is doped with 10 vol.% gallium oxide, again suggesting no cristobalite formation. According to the thermal expansion results shown in Fig. 8, the TCE from room temperature to 200 “C can be determined. It is found that the TCE decreases from 27 X 10m6 K-l for the gallium oxide-free sample to 3.5 X lop6 K-’ for the sample with 10 vol.% gallium oxide. Samples used for dielectric measurements were fired at 900 “C for 4 h. The glass composites are uniform and have a
I % I
Fig. 8. Thermal expansion as a function of temperature for the samples with 0 and 10 vol.% gallium oxide, and the inset showing the thermal expansion curve of pure cristobalite.
J.-H. Jean et al. /Materials Chemistv and Physics 42 (1995) 56-61
relative sintered density greater than 98%. It is found that the sample with 10 vol.% gallium oxide has a dielectric constant of 5.14 and a dielectric loss of 0.38% at 1 MHz.
With a sufficient amount of gallium oxide present, the cristobalite formation observed in an initially amorphous lead borosilicate glass is completely inhibited. The required gallium oxide content to prevent the formation of cristobalite decreases with increasing temnerature. The resulting cristobalite-free glass composite ha’s a thermal expansioi coefficient of 3.0-4.0 X 10e6 K- ’ in the temperature range of 2s 200 “C, and a dielectric constant of 5.14 and a dielectric loss of 0.38% at 1 MHz.
[l] W. Espe, Materials ofHigh Vacuum Technology, Vol. 2, Pergamon, Oxford, 1968, Ch. 10. [21 R.R. Tummala, J. Am. Ceram. Sot., 74 (1991) 895. [33 D.M. Mattox, S.R. Gurkovich, J.A. Olenick and K.M. Mason, &ram. Ena. Sci. Proc.. 9 (1988) 1567.  J.-H. Jean, unpublished results. [51 W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, Ch. 4.  J.-H. Jean and T.K. Gupta, J. Am. &ram. Sot., 76 (1993) 2010. 171 J.-H. Jean andT.K. Gupta, J. Am. Ceram. Sot., 76 (1993) 751. [81 J.-H. Jean and T.K. Guota. J. Mater. Res.. 8 (1993) 356. i9i J.-H. Jean and T.K. Gupta, J. Mater. Rex.: 8 i 1993) 1767.
Funding for this study has been provided by the National Science Council of the Republic of China under Grant No. NSC 84-0404-E-007-07 1.