NBSIR 74-510

Effect of Phase Separation on the

Physical and Chemical Properties of

Glasses-Density and Chemical Durability

Joseph H. Simmons, Susan A. Mills and Barbara F. Howell

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1974

Final Technical Report

Prepared for Office of Naval Research

Contract Naonr 12-73, NR 032-536

Department of the Navy

Code 471

Washington, D. C. 20360

NBSIR 74-510

EFFECT OF PHASE SEPARATION ON THE

PHYSICAL AND CHEMICAL PROPERTIES OF

GLASSES-DENSITY AND CHEMICAL DURABILITY

Joseph H. Simmons, Susan A. Mills and Barbara F. Howell

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

July 1974

Final Technical Report

Prepared for Office of Naval Research

Contract Naonr 12=73, NR 032-536

Department of the Navy

Code 471

Washington, D. C. 20360

a M.

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

1 . Irit roducti oh

A number of technical glasses either undergo liquid-liquid phase

separation in their annealing range or have compositions bordering on miscibility gaps. Some of these find extensive applications in the

chemical ware and window glass industry, and while their usefulness for

normal duty seems not to be greatly affected by the phase transitions,

it is likely that special applications such as long anneals or long

exposure to corrosive environments will dictate heat treatments with a combination of times and temperatures favorable for the development of

large internal structure. In order to better understand the processes

involved and to characterize the related changes in physical and chemical

properties, an investigation of the results of heat treatments which

maximize the phase separation effects in these glasses was undertaken.

1-4

In a previous series of reports we considered the mechanism of

phase separation and its effects on viscous flow processes in two borosilicate glasses of which one is commonly used for chemical ware. The

results are described and^analyzed in detail in our Annual Technical

Report of September 1973, and in a paper in the Journal of the American

Ceramic Society. We found that extensive phase separation occurs in

both glasses near the annealing region and the viscosity undergoes

increases 10,000 to 100,000 fold during an isothermal heat treatment.

In the analysis of these results, a theory was developed which accounts

for the viscosity change in terms of micros trueture development in the

molten glass.

The large increase in viscosity upon phase separation will affect

annealing schedules by changing the rate of stress-release. For example, one

off the glasses investigated. Type I in Refs. 1 and 2, has a complex

time and temperature dependence for viscosity in the annealing range.

This is demonstrated in figure 1 where the isothermal viscosity at

600°C is higher than at 550°C for times between 10 and 100 minutes.

This unusual behavior indicates that isothermal heat treatments at

560 °C will release internal stresses at a faster rate than at the higher

temperature for treatments shorter than 100 minutes. At 560°C, the

mi c restructure development is slower than at 600°C providing an additional

benefit for the lower temperature anneal. If a long heat treatment is

needed, then an increase in temperature to 600° or 620°C after 100 min.

will be desirable.

Having characterized the microstructure development and the stress

relaxation mechanism, we have proceeded to measure the effect of pressure

on phase separation, and to measure the loss of chemical durability

accompanying the two-phase fomation. The potential effect of pressure

or stress on the thermodynamic conditions controlling phase separation

are reflected in the behavior of the density, refractive index and

thermal expansion coefficient. The chemical. durability of an alkali

containing glass can be measured by the amount of alkali released from

the glass when exposed to the desired chemical environment. Of the

1
two glasses studied in the early part of this work ' only Type II,

the glass commonly used for chemical ware, is suitable for this study.

Type I, the simple sodium-borosilicate glass suffers too large a loss in

chemical durability during phase separation to provide for a convenient

analysis by the means contemplated here. Further, its poor durability

will greatly complicate the density measurements since the density

liquids are likely to alter its chemical makeup. Therefore, below, we report measurements of density, index of refraction, thermal expansion

coefficient and chemical durability on the Type II glass only. The

stress relaxation time for this glass during phase separation is shown

in figure 2.
2. Experimental Section

2 . 1 Glass Samples

1 2 Ihe glass chosen (Type II in our previous work ' ) has a composition

consisting of SiO^, ^2*^3' ^^2*^ ^'"2'~'3 proportions:

81:12.6:3.9:2.4 by weight, as determined by chemical analysis. Its phase

transition temperature is 649°C where its viscosity is lO^poises. It

separates into a phase which is almost exclusively silica and alumina

with a volume fraction of 0,55. The second phase, rich in soda and

boron trioxide occupies a volume fraction of 0.45.

The samples were in the form of rectangles 0.1x0.5x1.0 cm for the

density and index of refraction tests and rods 1/4" by 2 cm long for the

thermal expansion coefficient tests.

2.2 Index Measurements

The samples were heat treated in a furnace with an inconel core. The samples were dropped into and out of the furnace, thus effecting a rapid temperature change in each case. The reproducibility of the quench

was checked by heat treating several samples at the same temperature and

comparing the index differences. No discrepancy was found between the

readings.^

A Grauer Refractometer was used to measure the index of refraction.

The data for isothermal heat treatments between 525° and 520°C are shown in figure 3.

2 . 3 Density Measurements

The samples were heat-treated in the same manner as for the index

measurement. The density was measured using the sink-float technique

by immersing the samples in a thermostatically controlled density liquid.

The latter was a mixture of Iso-Propyl Salicylate (38.1%) and

s-Tetrabromoethane (61.9%) The liquid mixture was calibrated with two

density standards giving the values 2.22000 at 27.396°C and 2.23250 at

21.495°C. This yielded the following equation:

4- lead Pt resistance thermometer calibrated at NBS with a constant current

of 1.0 ma through the thermometer.

The measurement was effected by slowly raising the temperature of

the bath ( . 0035 °C/min) and recording the temperature at which the sample

changed from a floating to a sinking condition. The sample was restrained

6 cm below the surface of the density liquid in order to avoid evaporative

(1)

where ^ is in q/cm and T in °C. The temperature was measured with a

3
cooling effects.

The accuracy of the measurement depends directly upon the density

st£u:idards which were certified to - .0005 gr/cm-^ by the manufacturer

but not verified by us. The precision of the measurement is approximately

i 3x10-6 q/cm^ or Ad/d = ± l.Sxlo"^.

Samples were measured and heat treated in succession. Several

samples were used for each isothermal study but none were heat treated

at more than one temperature. The resulting isothermal density data is

plotted as a function of he at- treatment time in figures 4 and 5.

2 .4 Thermal Expansion Coefficient Measurement .

The coefficient of thermal expansion was measured in a modified

beam-bending viscometer.^ The cylindrical samples were placed in an inconel box within a fujmace, and the expansion of the glass during

heating was measured by recording the position of a long silica rod

resting on top of the sample cylinder with the sensing element of a linear deflection transducer attached to the other end. A concentric

silica cylinder resting on the box was used to support the outer portion

of the transducer. This arrangement allowed a measurement of the difference between the expansion of the sample and an equal length of silica.

The furnace was heated at 2°C per minute and the temperature was

monitored with a Pt-10% Pt-Rh thermocouple touching the sample. Temperature gradients within the chamber were less than S'C. The transducer

output was recorded on the y-axis and the voltage generated by the thermocouple on the X-axis of a recorder. A typical expansion curve thus

obtained is shown in figure 6.

The glass samples were first heat treated between 565° and 620°C

for a variety of times, and then were tested for a measurement of the

coefficient of thermal expansion. The data is shown in figure 7.

2 . 5 Chemical Durability Measurement

The measure of resistance of glass to chemical attack is highly

subjective to the test performed. Therefore, we decided to use the

Q ASTM durability test. While the chemical processes involved in this

* R. P. Cargille Laboratories, Inc., Cedar Grove, N.J. 07009.

4
test are complicated, we chose it because of its wide use in industrial

plants and, therefore, the ease in relating the data to that obtained

in industry and the possibility of a comparison with other materials

,

The experimental procedure is described in detail in Refs. 8 and 9.

Briefly, the glass samples in the form of 1/4-inch rods were heat-treated

at 565°, 585°, 600°, 620° and 640°C for selected time periods, and crushed

to form powders of a desired size (40 to 50 mesh sieves) . The powder

was covered with water and autoclaved at 121 °C for 30 minutes. Subsequently,

the amount of alkali released in the water during autoclaving was determined with a dilute standardized sulfuric acid solution using methyl red

as indicator.

This technique gives a good account of the amount of material

leached out during the autoclave treatment since the boric and silicic

acids formed are expected to be small in content and weak compared to

the sodium hydroxide base. However, in order to measure the Si, B and

Al released, we are preparing to test the released solutions with an Atomic Absorption Spectrometer. The instrument has been ordered and

the measurements will be conducted next fiscal year. The results of the

titration test are shown in figure 8 and demonstrate a large (100 fold)

decrease in the resistance of this material to chemical attack when phase

separation occurs

5
3, Analysis Section

3 . 1 Pressure Effect

3.1.1 Theory

The volume change resulting from phase separation can be analyzed

to yield the change in phase transition temperature due to applied

pressure or stress on the glass. The transition temperature can be

expressed in terms of the mixing enthalpy and entropy and the concentrations of the phases by means of Regular Mixing Equations. We have

shown that, with proper choice of phase compositions, the miscibility

gaps of binary-'--^ and temaryl^ systems can be described by means of the

regular mixing concepts. If the glass only separates into two phases,

as we have observed for this particular material over the temperature

range 540° to 660 °C,-'- then the transition temperature^ can be expressed as

where f (yi) is a complex function of the molar concentrations of the

component oxides and AH is the enthalpy of mixing for the system. At

the critical composition of the system, the transition temperature

often called the critical temperature, is expressed as:

(3)

The critical composition is defined as the conposition where the separate

phases are at equal fractional concentrations. In most cases this corresponds to the glass which separates into phases of equal volvune

fractions. The critical temperature is the highest transition tenperature achieved by the mixture of the same phases in any possible proportion,

The effect of pressure applied at constant temperature on the phase

transition is found by differentiating ^ as follows:

(4)

Using a Legendre Transformation and defining the volume coefficient of

thermal expansion as ®^ =^^'V^T*)^ /V ' change in phase transition

temperature may be expressed as:
where £kv = v^^^ - v^j^^^^^ is the change in volume per unit mole at

•teinperature T during phase separation, and 6fil= ^^mix unmix

change in expansion coefficient. The spinodal temperature

^

is similarly

affected:

The expansion coefficient in glasses is generally a small number and,

in most cases, TAo/ 4C i • Therefore, only the first term of Eqs. 5 and 6 needs to be considered. The equations show that if there is

expansion during phase separation /f ^ » there will be a reduction in

transition temperature from the applied pressure, or positive stress.

7
3.1.2 Coefficient of Thermal Exoansion

In analyzing the change in coefficient of thermal expansion with

various heat-treatments, care must be taken to consider the density of

the material at the beginning of the test. The test generally begins at room temperature and, therefore, the initial state of the glass is a function of the preceding cooling treatment through its density. Since

the physical property of interest is the change in volume per unit of

matter rather than per unit volume, the coefficient of thermal expansion

must be normalized by the density in order to compare materials with

different thermal histories. In this context therefore, we seek the

change in density Ap with temperature:

The density of each sample was measured prior to the measurement of

coefficient of thermal expansion, and the change in density with temperature is plotted in figure 9. The data demonstrates no apparent change

with degree of phase separation, therefore the normalized thermal expansion coefficient is not significantly altered by phase separation.

13 Since one of the phases is nearly pure silica, with an expansion

coefficient near 0.5xlO~^/°C, it is expected that the expansion coefficient

of the individual phases will differ measurably. The average value of

the bulk sample remained constant during phase separation, indicating

that the dependence of the coefficient of therroal expansion on composition

is linear along the tie line. This, however, does not imply that there

will be no stress development during cooling of the two-phase structure,

since there are differences in the microscopic coefficients of expansion.

In fact

, ^|tresses are expected and have been observed in phase-separated

glasses. The effect that these stresses can have on strength may be

considerable and presents an interesting possible research project.

3.1.3 Discussion

By^gomparision witii the miscibility surface of the Na2O-B202-SiO2

system, the composition of interest in this paper appears to be

close to the critical value (number 34 in Ref. 15^. Further, the volume

fractions, as measured from electron micrographs, are nearly equal,

therefore, it may be assumed that Q aj in our samples.

This greatly simplifies Eqs . (5) and (6) to:

(7)

(8)

where Av(T) is a function of temperature.

8
The volume change per unit mole during phase separation was obtained

from the density measurements as follows:

where ^ and ^ are the densities of the mixed and unmixed systems,

respectively, ''a comparison of the refractive index and density data

at 565°C and 620°C, shows that the polarizability of the glass is not

affected by phase separation and is not appreciably temperature dependent.

Therefore, index data was also used to calculate the volume change. The

results are shown in figure 10 where both density and index measurements

are combined. The data show that Av is very small at the transition

temperature but increases linearly with (T -T) as the temperature is

lowered. The change in transition temperature is found by combining

the results of the density measurements with Eqs. (8) and (9). In this

material, the change is a decrease in transition temperature with applied

pressure due to the decrease in density, but the effect is s^all in

magnitude: is -7xl0~ °K/kbar at 660°C and -19xl0~ °K/kbar at

526 °C. Measurements of the density j^ange during phase separation in a soda-silica glass by L. D. Pye et al have shown no measurable change,

indicating a linear dependence of molar volume on composition. Phase

separation in this sampl^^would therefore be unaffected by the application

of pressure. It appears that the density change in soda-lime-silica

glasses will be large and negative, also corresponding to a decrease in

phase transition temperature with applied pressure. However, it is also

possible for the density to increase during a phase transition. This

will cause an increase in transition temperature with applied pressure,

leading to a number of interesting pr^^lems in glass product manufacturing.

Density data reported by Vashal et al on SiO^-Al^O^-CaO-MgO glasses

doped Tvith TiO indicates the possible occurrence of this effect. Calculations based on Eqs. (8) and (9) show that a 5% difference in density

between the mixed and unmixed states will lead to a change in transition

temperature of 10°C per kilobar of pressure.

3.2 Chemical Durability

The glass rods used in this test were large enough to prevent

reliable chemical durability measurements for heat-treatment times less

than 15 minutes. The large size, however, was desirable to reduce the

relative amount of surface material when the samples were crushed to

powder. An analysis of the microstructure developed in fibers of this

glass for heat-treatments shorter than 15 minutes proved very difficult,

even at 620°C, due to the very small size of the structure and a lack of

sufficient chemical differentiation between the phases to insure a proper

etch for replication. However, from the viscosity data, it appears that

relatively little structure develops in this glass for such short heattreatments .

9
For heat-treatments lasting more than 15 minutes, as can be seen from figure 8, the chemical durability of the glass decreased significantly with advancing phase separation at each temperature. However,

there is little temperature dependence.

Since chemical attack, or amount of leached alkali will be a sensitive function of available surface area and surface composition, it

is likely to depend strongly upon phase composition, volume fraction

and phase interconnectivity . However, it would appear to remain relatively

independent of microstructure size.

Figure 11 shows a series of micrographs reprinted from Refs. 1 and

2 for the 620°C isotherm. This is representative of other temperatures.

The microstructure size changes considerably, but both the phase volxame

fractions and interconnectivity remain relatively independent of heattreatment time and temperature. Since the volume fractions are nearly

equal for both phases and the interconnectivity is high, there is no development of isolated pockets of one phase as proposed by Mazurin et al

to account for their observed changes in chemical durability.

The data shows no simple dependence on microstructure size since

the durability varies little with temperature while microstructure size

varies exponentially (see Ref . 1) . There are no appreciable changes in

volume fraction or interconnectivity, therefore, only the change in

chemical composition of the phases can be the cause of the large deterioration in chemical durability observed in this glass during phase

separation. As the material separates into phases more and less durable

than the mixed state, the resultant average durability will decrease.

This conclusion indicates that the two phases achieve compositional

equilibrium relatively slowly due to the high viscosity. Since the

chemical makeup of the equilibrium immiscible compositions varies little

with temperature (a steep miscibility gap, as found in this system) , the

change in chemical durability will also be relatively temperature independent

as we found in our measurements. Unfortunately, the titration measurements

which were used in this test cannot detect a change in composition of the

extract, since the ratios of alkali to other components are not known.

A solution of this problem is achievable with a detailed chemical analysis

of the extract by such techniques as atomic absorption spectroscopy. We

are presently acquiring such an instrument and will study the problem in

the following year. Some further support to our assertion is found in

the analysis of the acid etches used to develop structure for replication

and subsequent observation with electron microscopy. As stated in Ref. 1,

stronger acid solutions were needed for samples treated at the shorter

times which indicates smaller composition differences.

10
4. Siinunary and Conclusions

An equation was derived to describe the change in phase-transition

temperature due to pressure applied isothermally . The equation is in

differential form and is useful in calculating the change in transition

temperature. An integration is possible to express the pressure dependence

of the transition temperature explicitly if the change in isothermal

compressibility can be measured.

Measurements of the coefficient of thermal expansion on a soda

borosilicate glass widely used in industry for chemical glassware demonstrated no change with phase separation and a new equation was proposed

for comparing measurements of thermal expansion coefficients in materials

with different heat-treatments. Changes in density and index of refraction

were observed and were analyzed in terms of a molar volume expansion

during phase separation. The volume expansion implies a decrease in

transition temperature with applied pressure. The magnitude of the

effect is small in this material, but it was calculated that a 5% change

in density during phase separation would lead to a change in transition

temperature of 10°C per kilobar of applied pressure.

The chemical durability of the same glass was observed to decrease

significantly (100 fold) with heat-treatment time. There was little

o.'spendence , however, on heat-treatment temperature. This data coupled

with an analysis of electron micrographs taken on samples with similar

heat-treatments suggest that the decrease in chemical durability arises

from changes in the chemical composition of the phases.

11
5. References

1. J. H. Simmons, S. A. Mills, A. Napolitano, D. H. Blackburn and

W. Haller, NBS Tech. Note 792. (Sept. 1973).

2. J. H. Simmons, S. A. Mills and A. Napolitano, J. Am. Ceram. Soc. 57 , 109 (1974).

3. R. Mahoney, G. R. Srinivasan, P. B. Macedo, A. Napolitano and

J. H. Simmons, Phys . Chem. Glasses (Feb 1974) , to be published.

4. J. H. Simmons, S. A. Mills and A. Napolitano, J. Noncrystalline

Solids 14_, 302 (1974) .

5. S. Spinner and A. Napolitano, J. Res. NBS 70A , 147 (1966).

P. B. Macedo and A. Napolitano, J. Res. NBS 71A, 231 (1967).

6. "Test for the Density of Glass by the Sink-Float Comparator,"

Test C 729, Annual Book of ASTM Standards (American Soc. for

Testing and Mat., Philadelphia, 1973).

7. A. Napolitano, J. H. Simmons, D. H. Blackburn and R. E. Chidester,

J. Res. NBS (to be published).

8. "Resistance of Glass Containers to Chemical Attack," Test C 225,

Annual Book of ASTM Standards (American Soc. for Testing and Mat.,

Philadelphia, 1973)

.

9. B. F. Kowell, J. H. Simmons and W. Haller , "Chemical Durability

Degradation of a Glass During Phase Separation" (in preparation)

.

10. J. B. Thompson, Jr., Researches in Geochemistry 2_, 340 (1967).

11. W. Haller, D. H. Blackburn and J. H. Simmons, J. Amer. Ceram.

Soc. 57_, 120 (1974) .

12. J. H. Simmons, "Theoretical Analysis of Miscibility Gaps in

Ternary Systems" (in preparation)

.

13. E. A. Porai-Koohits , V. I. Averjanov, V. V. Golubkov and A. P. Titov,

Mat. Res. Bull. 7_, 1323 (1972).

14. F. Schill, Epitoanyag 24_, 453 (1972).

15. W. Haller, D. H. Blackburn, F. E. Wagstaff and R. J. Charles,

J. Amer. Ceram. Soc. 53_, 34 (1970) .

16. L. D. Pye, L. Ploetz and L. Manfredo, J. Noncrystalline Solids 14 , 310 (1974) .

12
17. I.S. Patel and S. M. Ohlberg, J. Appl. Phys . 43_, 1636 (1972).

18. B. G. Varshal, N. M. Vaisfeld, G. B. Knyazher and L. M. Yusim,

"Investigation of the Phase-Separation Processes in TitaniumContaining Glasses in the System Si02-Al203~CaO-MgO, " Phase

Separation Phenomena in Glasses, ed. E. A. Porai-Koshits

(Structure of Glass, vol 8; Consultants Bureau, New York, 1973)

.

19. N. P. Danilova, 0. V. Mazurin and T. S. Tsekhomskaja

,

IX International Congress on Glass _1, 825 (1971) .

13
6. Figure Captions

Viscosity of soda borosilicate glass plotted as a function

of heat-treatment time during phase separation at two

temperatures. Note that due to the complexity of the

effect, the viscosity at 600°C is higher than at 560°C

for treatment times from 10 to 100 minutes.

Stress relaxation time plotted as a function of inverse

temperature

.

Dependence of refractive index on heat-treatment time

for a variety of temperatures during phase separation.

Density and index data at 565 °C.

Density and index data at 620*'C.

Typical data from measurement of coefficient of thermal

expansion.

Coefficient of thermal expansion of samples heat-treated

at 585 and 620°C for the times shown.

.

Change in chemical durability for samples heat-treated

at temperatures below the transition temperature, from

565 to 620°C for the times shown.

Plot of normalized coefficient of thermal expansion for

samples heat-treated at 585 and 620°C for the times

shown

.

Figure 10 - Change in molar volume during phase separation plotted

as a function of temperature. The circles represent the

density measurements and the crosses, the refractive

index measurements.

Figure 11 - Micrographs showing the structure developed during heattreatments at 620°C for

(a) 8 hours

(b) 48 hours

(c) 192 hours

(d) 768 hours

The bar is 1/2 ym.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

14

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V
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NBS-1 UA (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. m HLle A I ii OH li l- l^ORT \'0.

NBSIR 74-510

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AM ' SUBTITLE

EFFECT OF PHASE SEPARATION ON THE PHYSICAL AND CHEMICAL

PROPERTIES OF GLASSES-DENSITY AND CHEMICAL DURABILITY

5. Publication Date July 16, 1974

6. Performing Organization Code

313.02

7. ALITHOR(S)

Joseph H. Simmons, Susan A. Mills and Barbara F. Howell

8. Performing Organ. Report No. NBSIR 74-510

9. PHRF0RM1N<: ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

10. Project/Task/Work Unit No. 313047F

11. Contract/Grant No. Naonr 12-73

NR 032-536

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Office of Naval Research

Department of the Navy

Code 471

13. Type of Report & Period Covered

Pinfll TprVinif-fll Rppnr-f14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary ol most signilicant information. If document includes a significant

bibliography or literature survey, mention it here.) This report covers results from density and chemical durability measurements on a borosilicate glass widely used commercially for chemical glassware while the glass

undergoes liquid-liquid immiscibility . A net decrease in density is observed during

an isothermal heat-treatment below the transition temperature. This density or molar

volume change is related to the effect of pressure on the phase transition temperature

by means of equations derived herein. The chemical dtirability of the material is

measured following the ASTM titration technique and shows a large degradation of

chemical resistance to attack by water as a result of sub-immiscibility structure

development. The effect is analyzed in terms of the change in composition of the

phases associated with the immiscibility transition.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Borosilicate glass; chemical durability; density; immiscibility.

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