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[TODOROKI, Shin-ichi](https://orcid.org/0000-0003-3986-1900), INOUE, Satoru, MATSUMOTO, T.

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[Combinatorial evaluation system for thermal properties of glass materials using a vertical furnace with temperature gradient](https://mdr.nims.go.jp/datasets/fb08c7a6-0b8a-44ba-9008-1ab4473462da)

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Combinatorial evaluation system for thermalproperties of glass materials using a vertical furnacewith temperature gradientS. TODOROKI∗, S. INOUE and T. MATSUMOTONational Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki305-0044, JAPANAbstractCritical cooling rateQ for zinc tellurite glass system is determined based on the time-temperature-transfer (T-T-T) diagrams, which are compiled by analyzing the crystallizedarea in the glass sample libraries annealed simultaneously by a furnace with temperaturegradient. This method reduces the laborious routine work for preparation, which is neededin the conventional method. Since the surface/volume ratio of the present samples is large,their crystallization is mainly governed by heterogeneous nucleation. Thus, theQ valuesin this study can be used as a practical index for the glass products whose surface shouldbe free of being ground and/or polished, such as fire-polished lenses, optical fibers andwaveguides.Key words: glass, crystallization tendency, critical cooling rate, time-temperature-transferdiagramPACS:06.60.-c, 06.60.Ei, 07.20.Hy, 65.90.+11 IntroductionThe methodology known as combinatorial chemistry has been widely used in thefield of organic chemistry such as medicine to bring about fruitful results in itsresearch and development. Recently, it is shown that this method is also applicableto the R & D of inorganic materials and devices whose form is thin film(See otherarticles in this volume). This paper reports about an expansion of this applicabilityinto hot melt processes which are typical and important in glass industry.∗ Corresponding author.Email address:todoroki@nirim.go.jp (S. TODOROKI)..Preprint submitted to Elsevier Preprint 23 October 2000Evaluation of crystallization tendency of glass materials is important for manufac-turing optical device such as optical fiber and waveguide, because the precipitatesact as a origin of light scattering and fracture. The most quantitative informationfor this is obtained by T-T-T(time-temperature-transform) diagram, which showsthe annealing temperature dependence of the time to precipitate(See the bottom ofFig. 1). This diagram shows how fast the melt should be quenched to avoid crys-tallization, but it needs laborious work to make a single diagram. The process isillustrated in the upper left of Fig. 1. In order to simplify the making process of T-T-T diagram, we propose an alternative method using a special furnace for makingglass sample libraries and performing successive heat treatment under temperaturegradient.2 ExperimentalZinc tellurite glass melt (xTeO2-(100−x)ZnO, x=90∼65 mol%) in a Pt crucible ismounted at the bottom of the furnace where the temperature is kept at 800◦C. APyrex glass capillary (75cm× 8mm OD× 1.5mm ID ) is inserted into the furnacefrom the top so as to be heated under a temperature gradient of800 ∼ 300◦C in50cm-span. The glass melt is sucked into the capillary by a vacuum pomp con-nected with the top of the capillary. The melt stops rising within 1 sec. because itstemperature decreases and its viscosity increases along the pipe. After a heat treat-ment for the prescribed time (50∼ 4000 sec), the capillary is pulled out at a fixedspeed and crystallization area is determined by eye.The temperature profile inside the furnace is determined by using a thermo-couplemounted inside the glass capillary. The change of the temperature inside the pipeduring pulling out is also recorded.3 ResultsThe glass inside the capillary includes some bubbles and fractures due to a shrink-age during the quenching of the glass melt whose expansion coefficient is largerthan that of outer Pyrex glass. No library is broken, however, because the pipe wallis thick enough. White crystallization area is clearly determined as a function ofannealing temperature which is shown as closed rectangles in Fig. 2.Sometimes it is observed that any devitrification does not occur even after the timewhen crystallized sample is once obtained. In such a case, re-examination is per-formed whether it crystallized accidentally or not. The accidental crystallizationarea is shown as open rectangles in Fig. 2.2Critical cooling rate,Q, is given as the slope of the line connecting between thepoint of melting state (t=0 sec,T=800 ◦C) and the nose of crystallization area,that is, the earliest point when crystallization is observed. DeterminedQ values areprinted in Fig. 2 and plotted in the phase diagram of TeO2-ZnO system[1] shown inFig. 3 (4, • and5). The temperature variation at the bottom of the capillary duringpulling out from the furnace is also shown as a curve in the Fig. 2. The quenchingrate for this is about 2.0 K/s. Thus, the melt cannot be quenched at the rate fasterthan 2.0 K/s. For the samples ofx=60, 85 and 90,Q is expected to be larger than2.0 K/s (4 in Fig. 3) because some crystallization is observed within 50s. Forx=65and 70,Q is to be smaller than 0.1 K/s (=400K/4000s) because precipitation doesnot occur even after 4000s annealing (5 in Fig. 3).4 DiscussionCrystalline phase appears via nucleation and growth processes. The temperaturewhere the nucleation rate (betweenTg andTx) is maximized is, in general, lowerthan that where the growth rate is maximized (∼ Tx). Thus, the quenched glass in-side the capillary is exposed to the crystal growth temperature range before stayingthe nucleation temperature range. This means that the present method can excludethe chance to nucleate before annealing, which is inevitably included in the sam-ple preparation of the conventional method. (Compare the thermal history in bothmethods as shown in the middle of Fig. 1).Once crystallization occurs, the precipitated area seems to move as increasing theannealing time shown in Fig. 2. On the ground that the nucleation of super-cooledliquid is a phenomenon of non-equilibrium state, there is not enough time and vol-ume to distribute nuclei uniformly in the glass, that is, the time for nucleation iseliminated to a short time during a quenching period to the the annealing temper-ature just after the suction, and the volume of the sample is also eliminated to 1.5mmφ. In addition, fractures and bubbles in the glass distribute randomly along thelength, which are the main origin of heterogeneous nucleation. Thus, the determi-nation of transfered area in T-T-T diagram needs to be cautious to reach statisticallyadequate. In this study, the nose of crystallization area is determined after examin-ing several trials of same (or longer) annealing period in order to confirm it.It is possible to examine the validity ofQ of the present study by comparing theglass forming region determined before. A compositional dependence of criticalcooling rateQ is roughly given as a cooling rate dependence of glass forming re-gion, that is, a curve connecting the edges of the regions for different cooling rate.As shown in Fig. 3,Q values given by the present method(4, • and5) is differentfrom that estimated from their glass forming regions (+×, × and+)[2]. This is be-cause of the difference in mechanism of crystallization observed in these methods.3In Bürger’s work, the glass forming region is determined by casting 100g batchof melt into copper or graphite moulds. Because of the large size of the samples,their surface is quenched faster than the inner, which bring about noticeable errorin the corresponding cooling rates. At the same time, any surface crystallization issuppressed due to the faster quenching of the surface. On the other hand, the sizeof our samples is about 2 orders smaller than Bürger’s, so the surface/volume ra-tio is also larger and the temperature difference between their surface and insideis small. This means the crystallization of our samples is governed mainly by het-erogeneous nucleation compared with the Bürger’s. In general, activation energyof heterogeneous nucleation is lower than that of homogeneous nucleation. There-fore it is reasonable that critical cooling rate of our samples is larger than that ofBürger’s.It is concluded that theQ values obtained by the present method is closely re-lated with surface crystallization and, thus, provides useful information for the glassproducts fabricated without grinding and polishing, such as fire-polished lens, op-tical fibers and waveguides.5 ConclusionParallel annealing of long glass samples performed by the vertical ring furnace withtemperature gradient is useful for reducing the work required for making T-T-T di-agrams. Since the crystallization occurred in the samples is governed by heteroge-neous nucleation, this method provides unique and important information for theglass industry.References[1] M. Marinov and V. Kozhukharov,C.R. Acad. Bulg. Sci.25 (1972) 329.[2] H. Bürger, K. Kneipp, H. Hobert, and W. Vogel,J. Non-Cryst. Solids151 (1992)134.[3] W. D. Kingery, H. K. Bowen, and D. R. Uhlmann,Introduction to Ceramics, 2nd ed.(Jhon Wiley & Sons, New York, 1976).4Figure CaptionsFig. 1 Two procedures to make T-T-T(time-temperature-transform) diagram, con-ventional and proposed. In the diagram shown in the bottom, crystallized point isshown by closed area. The former includes 3 steps; (1) making many glass sam-ples(melting, casting and cutting), (2) annealing them at several temperatures fordifferent periods, and (3) examining them. The latter includes 2 steps; (1) anneal-ing a sample library in a temperature gradient furnace immediately after making itby sucking glass melt into a glass capillary, and (2) examining. Thermal history ofglass samples are shown in the middle.Fig. 2 T-T-T diagram ofxTeO2-(100− x)ZnO glasses prepared in this study. Eachvertical line represents one glass sample library. Each closed rectangle is the tem-perature range where visible precipitation is observed. Open rectangle is acciden-tally precipitated area (see text). Curve represents the temperature change at thebottom of the capillary pulled att = 0 which is determined by a separate exper-iment. Arrows are the characteristic temperature for each glass composition,Tm:melting temp.,Tx: crystallization temp., andTg: glass transition temp., which aredetermined by DTA measurements.Fig. 3 Critical cooling rate(4, • and5), phase diagram[1] and glass formingregion(+×,× and+)[2] of TeO2-ZnO system.4: > 2 K/s,5: < 0.1 K/s,+×: glasseswere made by twin-roller technique (> 10 K/s),×: casted into copper mould (∼ 10K/s), and+: graphite mould(1K/min).5TimeTemp.TimeTemp.Temp.Time TimeTemp.T1.....T-T-TdiagramConventionalThis studyT1T1 T2 T3 T4 T1 T2 T3 T4Fig. 1. Two procedures to make T-T-T(time-temperature-transform) diagram, conventionaland proposed. In the diagram shown in the bottom, crystallized point is shown by closedarea. The former includes 3 steps; (1) making many glass samples(melting, casting andcutting), (2) annealing them at several temperatures for different periods, and (3) examiningthem. The latter includes 2 steps; (1) annealing a sample library in a temperature gradientfurnace immediately after making it by sucking glass melt into a glass capillary, and (2)examining. Thermal history of glass samples are shown in the middle.63004005006007008000 500 1000 1500 2000Temperature T/�o C80TeO2-20ZnOQ=2.0K/sTgTxTm3004005006007008000 500 1000 1500 200075TeO2-25ZnOQ=0.5K/sTgTxTm3004005006007008000 1000 2000 3000Temperature T/�o CTime t/sec65TeO2-35ZnOQ<0.1K/sTgTxTm3004005006007008000 500 1000Time t/sec60TeO2-40ZnOQ>2.0K/sFig. 2. T-T-T diagram ofxTeO2-(100−x)ZnO glasses prepared in this study. Each verticalline represents one glass sample library. Each closed rectangle is the temperature rangewhere visible precipitation is observed. Open rectangle is accidentally precipitated area (seetext). Curve represents the temperature change at the bottom of the capillary pulled att = 0which is determined by a separate experiment. Arrows are the characteristic temperaturefor each glass composition,Tm: melting temp.,Tx: crystallization temp., andTg: glasstransition temp., which are determined by DTA measurements.7300400500600700800900100040 50 60 70 80 90 1000.01110010000Temperature (�o C)Cooling rate (K/s)�TeO2(mol%)TeO2-ZnOZnTeO3Zn 2Te 3O8Fig. 3. Critical cooling rate(4, • and5), phase diagram[1] and glass forming region(+×,× and+)[2] of TeO2-ZnO system.4: > 2 K/s,5: < 0.1 K/s,+×: glasses were made bytwin-roller technique (> 10 K/s),×: casted into copper mould (∼ 10 K/s), and+: graphitemould(1K/min).8