Thermal Expansion of Binary Alkaline-Earth Borate Glasses Herman F

Home , Barium borate

Journal of Research of the National Bureau of Standards Vol. 56, No.2, February 1956 Research Paper 2650 Thermal Expansion of Binary Alkaline-Earth Borate Glasses Herman F. Shermer

The thermal expansivity of a number of calcium, strontium, and barium borate glasses are reported. The ex pansivity of the calcium and strontium borates increases with increas ing alkaline-earth oxide concentration. The expansivity of the barium borate glasses is a m inimum at about 20 mole percent of barium oxide. The expansivity measurements on the glasses were correlated with the density measurements in the liquid range. The expansivity of any composition is the lovvest in the glassy (solid) region and is greater in the liquid region. The expansivity in a third region which was described as "very viscous" was not m easured but was indicated to be greater than in either of the measureable regions. The density curves for each alkaline-earth borate series converges with increasing temperature.

1. Introduction hours, after which the power to the furnace was shut off, and the glasses were allowed to cool to room A study of the thermal expansion of the binary temperature. Polariscopic examination of the speci alkaline-earth borate glas es was undertaken as part mens revealed a slight amount of strain in the cal of a comprehensive investigation of various physical cium borate glasses, but this strain was not detected properties of simple glass-forming systems. This in the expansion curve. The presence of strain i report is the third of a series on the relationships normally indicated by a decrease in the expansion between chemical composition and physical proper with increase in the temperature in the range just ties of the binary alkaline-earth borates. The first below the point at which rapid expimsion begins. report [1] 1 was mainly concerned with the properties The samples used in the determination of thermal of the molten compositions, such as viscosity and expansion were cut from cylinders (approximately surface tension, and the second [2] was concerned H in. in diameter and 3 in. long) that had been used with the compressibility of the alkaline-earth borate for compressibility specimens in previous work [2J. glasses. Bridgman and Simon [3] have shown that a com The plan of the present work was the determina paction of fused B20 3 occurs during compression tion of the relative eff ects of varying amoun ts of measurements. As the glasses used in this study calcium, strontium, and barium oxides on the thermal had been used previously for compressibility meas expansion of borate glasses. It was desired to stud y urements and contained 63 to 84 mole percent of these relationships over as wide a composition range B20 3 , one must consid er the possibilitv that such as possible, but the region of glass formation was compactions may have occurred. Weir and Shartsis limited at low alkaline-earth oxide concentrations [2] ~eport a decrease in volume during compression by the formation of two immiscible liquids, and at testll~g oJ? the order of 0.2 percen t ltn d a.1so repor~ it high concentrations by crystallization [4 , 5]. as bemg mdependent of B20 :1 COl1 centra,tlOtl . An m crease of this magnitude was attributed t.o experi 2. Preparation of Glasses and Method of mental error. Even if this compaction were real it Test is believed that its magnitude is too small Lo sf'r iou ~ly affect the expansion data. Bridgman and ~imol1 l3] The glasses investigated were made by m elting have indicated that time and/or tf'mperature may the required amounts of reagent-grade chemicals in be employed to restore the specimens to tht\ir original platinum crucibles at approximately 1,3 00 ° C. densities. In the present investigation it is believed The alkaline-earth oxides were introduced to the that the period between the compressibility measure batch in the form of carbonates, and the boric ments and expansion measurements (ll Pproxima tely oxide was added as boric acid. The melts, which 6 months) and the heatin g of the specimetls to 200° C weighed approximately 500 grams, were stirred with preparatory to measurement minimized any possible a propellor-type platinum stirrer until they appeared eff ect of the previous compression measurements. clear (at least 1 hour), and then they were poured An adjacent portion of the specimen was removed into a cold iron mold. These binary glasses were and analyzed by triple evaporation with HF and later annealed by holding them at 590° C for 3 hours H2S0 4 . 2 and then cooling them at 3 deg C per hour for 30 • The compOSitions w~r~ analyzed by Ernest B. Clark, formerly of th~ stall I Figures in brackets indicate the literature references at the end of this paper. of tbe National Bureau of Standards.

73 Following the original determination of thermal -. 110 ,---r------,---,----.---.----.----.-----, expansion, an adj acent portion of the four samples of calcium borate glasses that showed strain was reannealed to determine what differences, if any, would be observed in the thermal-e'{pansion curves. These glasses were reannealed by heat.ing them to 100 655 0 C for 3 hours. They were then cooled at ap proximately 3 deg C per hour until they reached 455 0 C (68 hours), after which thr.y were allowed to cool in the sealed furnace, with no further applica tion of power. 90 The measurements of thermal expansion were made by the interferometer method of Pet.ers and Cragoe [6] as modified by Saunders [7]. The test specimen was formed into a T-shaped spacer about 5 mm high, which was placed between two optically 80 flat Vycor disks, the lower one of which served as an interference thermometer. This "sandwich" was placed in a metal cup with a fused-silica top to de 37.1 MOLE "10 Co 0 _ _ 34.9 crease any temperature gradients. This cup was E MOLE u ~C oO placed within a resistance-type electric furnace, " 70 Vl which was heatrd at 2 deg C per minute from room Z o 31.4MOLE temperature to above the deformation point. The

74 TABLE l.- Thermal expansivity of binary alkaline-earth bomte glasses

Chemi cal composition A verage coefficients of ex pansion per degree Cat- a Dpnsity I e D e[or- (room I ntlCul mation temp) temp temp no RO Room to ]00° e 1 1000 to 200° e 1 2000 to 300° e 1 300° to 400° e 1 400 0 to 500° e 1 .500° to 600° e 1 Fused B,O,

JVoie "l, wt% g/cm' 1 °C °C 0. 0 0. 0 I. 859 260 305 16. 16 X lO-6 1 13. 77XlO-'1 __ ------______1_ _------1------1------1 1 1 1 eaO-B,O,

6 7. 16 XlO-

SrO- B,O,

2t.4 29. 0 2. 619 598 614 5. 79XlO-6 6. 22 X 10- 6 6. 66XlO-6 7. 28X IO-

BaO-B,O,

17.3 31. 6 2. 678 509 540 6. 59 X 10- 6 6. 69X IO-6 7. 25 X IO-6 7. BOXlO-6 9. 77 X IO-6 -. ------20. 1 3.'.8 2. 813 56l 5BO 6. 19 6.59 7. 0l · 7.67 8. 39 ------23.7 40.6 2. 996 575 595 6. 43 6.64 7. 02 7. 71 8.21 ------.--- 27.1 45. 1 3. 190 589 606 6. 59 6. 90 6.01 8.14 9.36 ------_.- 29. 0 47. 5 3. 289 587 606 6.82 7.46 7.89 9. '18 10.39 ------3t. 7 50. 6 3.429 _,98 (\10 7.37 7.64 8.13 8.86 9. 58 ------36.0 55. 4 3.630 594 609 7.86 8.48 8.91 9.68 ]0. i4 ------

a rrhe estimated accuracy 0f values of average coefficient of expansion p f'.r degree C is ± O.l X 10- 6, The values gl ven contum one ndchtlOnal fi gure [or con vo n lene\:" n intercom parisons and better determ ination of greater tem perature ra nges.

3.2. Calcium Borates calcium borates are includcd in table 1. Values for the l'eannealed samples ar c also given. In gene~'al , 'fhe results of the thermal-expansion measure the l'eannealed glass had a slightl:v lower expanSlOn ments of the calcium borate glasses are given in table below the critical temperature. It may also. be 1 and fio'ure 1. Although the four curves for the noted that the height of the curve at the defol'ma:tlOn calcium borate glasses are similar, certain trends are point was greater in the reannealed sample, mdlCat apparent. The most obvious of these is the iJ?crease ing a higher density at room temperature. The sam in expansivity with increasing concentratlOn of ples used for r eannealing were not large enough for calcium oxide. This is true in the region from room accurate density determinations. . . temperature to the "critical" temperature and also The apparent increase in density of the calcmm at the deformation point. The critical temperature borate glasses when reannealed at 655 0 0 indicated is defined here as the approximate temperature at that the ori O'inal annealing at 590 0 0 was not con which rapid expansion begins. This is. in contrast tinued for s ~fficient time for equilibrium conditions wit h the expansion of fused B 20 a, WhICh In,ay be to be established at that lower temperature. It also thought of as having the lowest .conc entratlO~ of indicates that the equilibrium condition of the glass calcium oxide, and which has the highest expanslOn. before and after the original annealing corresponded It ma \' also be noted that the temperature of the de to a temperature above the equilibrium condi~ion forma"tion point increases with increasing calcium that was established during the second annealmg. oxide concentration. The deformation point is the highest point on an expansion curve obtained by the The results of the original measurement of thermal interferometer method and corresponds to the tem expansion are shown in figure L The results of the perature at which viscous flow exactly counteracts second determination are in table 1 but because of thermal expansion. the small differences in expansivity found upon Thc valu es of the coefficients of expansion of the reannealing the curves are not shown.

75 8 0 r----,----,----,----,----,----,----r----,

o~ __~ 3~7.~I~M~0~LE~'~~~C~0~0 ____ 2.6 o~------:;3-:-4 .-:-9-:-M:::0:-CLE;:-":C-~--:C-oO;O--- - 33.3 MOLE "I, SrO -----~II 7 0 29.3 MO LE % CoO 2.4

~ 26.0 MOLE % SrO --_ 28.5 MOLE E -%5rO u.... '" 2.2 - 6 0 ::;; - ~ 2.0 z E '"w u o ..... 50 If> Z o 33.3 MOLE % SrO 1. 8 a: u ~

z 28.5 MO LE S rO FUSED 820, 'Y, 1.6 o 40 If> Z

33.3 MOLE 'Y, SrO

30.5 MOLE % SrO 36.0 MOLE 80 0 3.0 a % a 28.5 MOLE '/, 5 rO ~ 70 E 26.9 MOLE % SrO u a "- a Vl 2.8 2 9 .0 MOLE % 800 ::E 26.0 MOLE % SrO 2.1.8 MOLE % SrO a r- a >- 21A MOLE % SrO iii 2.6 60 z 2 0 . 1 MOLE % BoO w 0

2.4 E u , 5 0 (j') z o 2.2'----J.--'-----'---'--- '------'- --'---'---'---'-----J.--' a: u o I~O :::;; TEMPERATURE, 'C , z FIGURE 4. Densities of some binary s~r ont ium boraw glasses ~ 40 and liquids as a function of temperature. z

3.2 0-----______

~ E 27.1 MOLE % 800 u

Vl ~ 3.0 0---______10 cr 23.7 MOLE % 800 > >- ~ 2.8 0 ____2_0_.I __MO_L_E_'Y...::., .,::8_00 __ w o O ~--~----~--~-----'------'----~------'--~ 0---______17.3 MOLE % 800__ o 800 TEMPER AT U RE , ' c 2.6 FIGURE 5. Linear thermal expansion of four selected binary barium borate glasses.

2.4 the mlmmum. rrhis minimum is better seen in figure 7. Although the expansivity of the barium borate glasses, below the critical temperature, passed through a minimum in respect to composition, the total expansion from room temperature to the defor TEMPERATURE, 'C mation point did not. It may be noted here that FIGURE 6. Densities of some binary barium borate glasses and the minima in the values of the coefficients of expan liq1iids as a function of temperature. sion with concentration do not occur at the same point in the various temperature ranges. In general, in alkaline-earth oxide concentration is in the same there seems to be a translation of the minimum to relative direction as the decrease in expansivity higher barium-oxide concentrations with increase in between fused Bz0 3 and the barium borate glass temperature. containing the least amount of BaO. The regions Figure 6 shows the relationship between densities, of glass formation of the calcium and strontium or expansivities, of the binary barium borates with borates do not contain minima because of their small temperature. The characteristics of this set of compositional range. The glass-forming region of curves are similar to those discussed previously for the barium borates, however, is longer and contains the calcium and strontium borates. 77 15 r---,----,----,----.----,----,----~--, 700,----,----,---,----,1 1----,----,1----,----,

Ca~ ~sro 13 ~ 600 - 80 0 - ..o w a: ::J t- 20

I I I 300 0L----L----ILO ----~--~20~--~--~30~--~--~40

RO , mole% RO , mole '7'.

FIGURE I . Coefficient of linear thermal expansion as a function FIGURE 8. Deformation temperatul'e as a function of com posi of composition of binary alkaline-earth borate glasses and tion of binary alkaline-earth borate glasses and fused fused B 20 3• B 20 a• (6, calcium borates; 0, strontinm borates; and 0, barium borates.) (6, calcium borates; 0, strontium borates; and O. barium borates.)

3.5. Comparison of Therma l Expansivity of Alka line that are applicable to the present investiga tion, Earth Bora tes namely, B 20 a, OaO, and BaO. The values assigned to these oxides by various investigators leave much A comparison of the mean coefficient of expansion to be desired. Figure 7 shows that the expansion over the range from room temperature to 200 0 0 of versus-composition curves of the binary alkaline the binary alkaline-earth borate glasses as a function earth borate glasses are not linear, and, therefore, it of concentration of alkaline-earth oxide in mole per would seem that in tIns case the coefficient of thermal cent is shown in figure 7. It is apparent that a min expansion is not an additive property. imum would have to occur in each of the curves if glasses could be made with lower concentrations of alkaline-earth oxides. There is not sufficient infor 5. Coordination of B20 3 mation available to determine the location of the minima in the calcium and strontium curves. The thermal expansivities of alkaline-earth borate Figure 8 shows a comparison of the deformation glasses are of special interest because they exhibit the temperatures of the binary borate glasses as a func so-called " boron anomaly"; i. e., they show minima tion of a.lkaline-earth oxide concentration in mole with respect to composition. Warren and Pincus percent. These curves show that at comparable [12] offer an explanation for this anomaly. It con·· compositions the calcium borate glass has the highest cerns the ability of the boron atom to change from temperature at its deformation point, followed by triangular to tetrahedral coordination when an the strontium and then the barium composition. oxide such as OaO is present and supplies the neces This is in the same order as the melting points of the ~ary oxygen. It is believed that fused B 20 a is tri three alkaline-earth oxides. angularly coordinated, and that with the addition of OaO, increasing amounts of boron change to tetra 4 . Ca lculation of Thermal Expansion From hedral coordination. Stegmaier and Dietzel [13J Composition demonstrated that beyond a point determined by the particular oxides present, any additional content Much work has been done in the past in an effort of alkali or alkaline-earth oxides will tend to reverse to find a series of factors from which thermal expan the process, i. e., change the B 20 a from fourfold to sion could be predicted from the composition of vari threefold coordination. A mathematical approach ous glasses. .Sun and Silverman [11] compiled the to the problem such as that of Warren and Pincus results of several of these investigations. Among requires more X-ray data on the particular system s the oxides included in their compilation are three of glasses under study than are now available. An 78 j altel'llate approach is from the measurements of 7. References physical properties. The measurement of thermal rxpan ion serms to be quite useful in this respect. [11 Leo Shartsis and H . F. Shermer, Surface tension, densit,v. The minima in the thermal-ex-pansion-composition viscosity, and electrical resistivity of molten binary alk a l ine ~ earth borates, J . Am. Ceram. Soc. 37, 544 curves hown in figure 7 are interpreted as indicating (1954) . maxima in the tetrahedral coordination of B 20 a. [21 Charles E. Weir and Leo Shartsis, Compressibili ty of This seems to be in accord with the fact that the B04 binary a lkaline-earth borate glasses, J . Am. Ceram. tetrahedra ten d to strengthen the structure and Soc. (submitted for pu blication). [31 P. W . Bridgman and 1. Simon, Efrects of yery high thrrdore have a lower expansion. pressures on glass, J . App!. Phys. 24, 405 (1053). [41 E. P . Flint and L. S. Wells, The system li m e ~bori c oxide silica, J . Research NBS 17, 745 (1936) RP941. 6. Summary and Conclusions [5) Ernest M . Levin and George M . Ugrinic, The system barium oxide-boric oxide-silica, J . R esearch NBS 51, The thermal expansivity of glasses formed in the 37 (1953) RP2430. three binary alkaline-earth borate series was meas [6) C. G. Peters and C. H . Cragoe, Measurements on the thermal dilation of glass at high temperatures, BS ured. The thermal expansivity of the glasses in Sci. Pap. 16, 449 (1920) S393. creased with increasing concent,ration of calcium or [7) James B . Saunders, An apparatus for photographing strontium oxide. The expansivity of the binary interference phenomena, J. R esearch ?\BS 35, 157 (1945) RP1668. barium borate glasses went through a minimum with [8) G. W. Morey, Properties of glass, 2d ed., p. 273 Am. changing concentration of barium oxide. A mini Chem. Soc. Monograph No. 124 (Reinhold Publishing mum was implied in the other curves by the high Corp., New York, N. Y., 1954). relative expansion of fused B20 a. It was not found [9) M . D . Karkhanavala, Bibliography of thermal expansion possiole to obtain additive factors for the calcu lation of glasses, Glass Ind. 33, 404 (1952). [10) Ja mes J . Donoghue and Donald Hubbard, Thermal ex of thermal expansion that would be applicable to pansion studies of boric oxide glass and crystall ine l these compositions. boric oxide, J. Research NBS 27,371 (1941) RPl+25. I The results were correlated with the density values [11) Kuan~Han~SLln and Alexander Sil verman, Additive obtained previously in the liquid range in order that factors for calcul ating the coefficient of thermal ex pansion of glass from its composition, Glass Ind. 22, the relationship between expansion and temperature 114, 125 (1941). eould be studied over a greater temperature range. [l2) B. E. vVarren and A. G. Pincus, Atomic consideration of The expallsivity of the solid glasses was the lowe t, immiscibili ty in glass systems, J . Am. Ceram. Soc. 23, and that of the measurable liquids was m uch higher. 301 (1940). I [13) V·l . Stegmaier and A. Dietzel, Die Bedeutung cler The change in density across the very viscous region Ba izitiit von Gla schmelzen und VersLlch e zu deren ~ was such as to show a still greater expansivity in Me sung. T eil II, Glastech. Ber. 18, 353 (1940). this range. The density curves were found t'o con ,~e rge with increase in temperature. WASHINGTON, November 1,1955.