Direct Bonding of Basic Brick by BEN DAVIES and FRANK H. WALTHER

Garber Research Center, Harbisan-Walker Refractories Company, Pittsburgh, Pennsylvania

Direct bonding in burned magnesite-chrome re- fractories is described as a mineralogical phe- nomenon and as a factor governing brick proper- ties. Hot-load tests are recommended as a supplement to hot modulus tests to obtain a meaningful picture of high-temperature strength. The importance of purity of ingredients is thus made more apparent. Direct-bonded brick from open-hearth service are shown to have a lower level of slag absorption. The result was repro- duced in a unique laboratory diffusion furnace and a relation between silicate buildup and

original brick purity is established.

I. Introduction N TllE past five years the conversion of open-hearth roofs from silica to basic has been spectacularly effective. Initially the aim was to establish economic equality, but

furnace operators soon began to expect service well beyond tlie break-even point. The change was accomplished pri- marily by the use of chemically bonded brick composed of niagnesite and chrome ore.

Burned brick played a minor role in the conversion, but in the past year or so several installations of burned magnesite- chrome brick have given campaigns equivalent to the more successful applications of chemically bonded basic brick. Now a new period of service testing has started with the introduction oT high-fired, high-purity, direct-bonded brick.

Burned basic refractories made from a combination of magnesitc and chrome ore can be broadly classified as three- phase systems composed of (1) magnesia, (2) spinel, and (3) silicatcs. Depending on the spatial relation among these phases, these refractories can be classified further as silicate bondcd or direct bonded. The term “silicate bonded” refers to the thin film of silicate minerals that surrounds and bonds together the refractory aggregate. The term “direct bonded’ describes the direct attachment of the magnesia to tlie chrome ore without any interrupting films ol silicate. The silicate- bonded basic brick are old, being first marketed in the early 1930’s. The direct-bonded brick are of much more recent origin. They have been iinrlteted in the United States in the last 0 months, although the earliest literature references to the principles of direct bonding appeared in the British literature in 1959-19GO’ and have been extended further by Hubble and Powers in the linited States.2

I

II. Raw Materials and Brick Manufacture Probably the most important single factor that led to tlie

commercial development of basic brick with a direct bond was the production of high-purity raw materials. Figure 1 shows the total impurities in brickmaking magnesites used in the greatest tonnage in 1945, 1950, and 1955 compared with the purest bricltmaking magnesite recently made available. In 1945 the total impurities in brickmaking commercial magnesites were commonly ahout 12%. Since then significant advances have been made in both chemical processing and burning techniques with the result that magnesite is now available with only 2% impurities and a density of 9470 of theoretical.

Steps also have been taken to procure chrome ores with a minimum of accessory oxides. Today several types of chrome ores are available with silica contents as low as 1.5 to 2.5%.

12 -

10 - 5 u)

I- E 3

w 8 - - 6 - I

-I

2 4 - e 2-

n - 1945 1950 1955 1963

Fig. 1 . Total impurities in the principal grades of brickmaking magnesites in 1945, 1950, and 1955 compared with the purest

grade available in 1963.

In the studies reported here, these high-purity raw ma- terials were used to manufacture magnesite-chrome brick using laboratory mixing, pressing, drying, and firing proce- dures. All the direct-bonded brick tested were of the same niagnesite-chrome composition. They contained about 75% MgO and their combined lime and silica content was about 2’%, except where it was purposely varied in the experiments on purity. All brick were burned a t the same temperature except in experiments in which this factor was the variable being studied.

111. Test Results

( I ) Effect of Burning Temperatures Probably the single most important property that charac-

terizes direct-bonded brick is high strength at elevated tem- peratures. High strength depends on the direct attachment of chrome ore and magnesia; the temperature a t which the brick are burned during manufacture is important in effecting this direct attachment. Figure 2 shows the modulus of rup- ture at 2300OF of high-purity magnesite-chrome brick which were burned at increasing temperature levels. Strengths ob- tained, may be d.ivid.ed into two general categories. Conven-

Presented at the Sixty-Fifth Annual Meeting, The American Ceramic Society, Pittsburgh, Pa., April 30, 1963 (Refractories Division, No. 11-R-63). Received June 27, 1963; revised copy received December 2,1963.

The writers are, respectively, manager, Basic Refractory Re- search Group, and senior mineralogist, Pctrographic Department, Garber Research Center, Harbison-Walker Refractories Com- pafy.

J. Laming, “Recent Work on Chrome-Magnesite Brick,” Refractories J . , 35 [3] 116-20 (1959); Ceram. Abstr., 1959, August, p. 208a.

( b ) H. M. Richardson, I<. Fitchett, and M. Lester, “Bond Structure and Behaviour of Basic Bricks at High Temperatures,” Trans. Brit. Ceram. SOC., 59 [ll] 483-504 (1960); Ceram. Abstr., 1961, September, p. 212d.

( c ) James White, “Recent Research in Refractories at the University of Sheffield,” Refractories J . , 36 [3] 60-74 (1960).

2 D. H. Hubble and W. H. Powers, “High-Fired Basic Brick for Open-Hearth Roofs,” Am. Ceram. SUG. Bull., 42 [7] 409-13 (1963).

( a )

116

March 1964 Direct Bonding of' Basic Brick 117 2000

LL

0 0 c) N

5 1000 Y LL 3 L

a 3

LL 0

a

Fig. 4. High-fired magnesite-chrome brick showing moderate amount of direct bonding. (Reflected light, X40.)

0

INCREASE IN BURNING TEMPERATURE +

Fig. 2. Hot strength vs. burning temperature for direct-bonded and conventional products.

Fig. 3. areas) surrounded by void space (black area) and magnesia (M).

Burned magnesite-chrome brick showing chrome grains (light-gray (Re-

flected light, X50.)

tional products, burned at lower temperatures, have modulus of rupture values in the range 300 to 500 psi at 2300'F. With higher burning temperatures the hot strength of the brick increases sharply. Brick with modulus oi rupture values above 1000 psi at 2300'F fall into the class of direct-bonded brick. The exact temperature which will transform a brick iroin a conventional to a direct-bonded product may vary, depending on composition as well as on a variety of manufac- turing techniques.

Figures 3, 4, and 5 are photomicrographs of high-purity magnesite-chrome brick that show how the microstructure and direct bonding change as the burning temperature passes through this transformation zone. Figure 3 shows a brick burned a t a temperature just below the transformation zone. The chrome grains are surrounded by voids and show little or no direct attachment to the magnesia. Figure 4 shows a brick burned just above the trans€ormation zone. It has the degree of direct bonding seen in refractories presently available on the market. Figure 5 shows a brick burned well over the transformation temperature in the laboratory. It shows a very high degree of direct bonding.

Fig. 5. High-fired magnesite-chrome brick showing extensive direct bonding. (Reflected light, X40.)

The formation of the direct bond between the chrome ore and magnesia is believed to result from a chemical union between these two phases caused by the interdiffusion and reaction of RO and RzOi groups. For this phenomenon to take place the chrome ore and magnesia must be in immediate contact. When the burning temperature becomes high enough, the silicate phase that normally intervenes between the chrome ore and magnesia diffuses into the groundmass and forms in discrete pockets. The chrome ore and magnesia form a chemical and mineralogical union.

(2) Strength Properties When the bond is formed in this manner, the brick have

excellent hot strength. This strength can be measured in a variety of ways, one of the most important of which is the hot modulus of rupture test. Figure 6 shows the modulus of rupture between room temperature and 2500°F of a direct- bonded high-purity magnesite-chrome brick with an MgO content of about 75%. For comparison, results are also shown for superduty silica brick and unburned magnesite- chrome brick containing about 65% magnesia which have been widely and successfully used in open-hearth roofs.

Silica brick long have been recognized for their outstanding strength a t elevated temperatures. Because of higher operat- ing temperatures, their replacement in open-hearth roois began in about 1957 with basic brick with much higher refractoriness but with lower hot strength. It long has been realized that the

118 Journal oJ The American Ceramic Society-Davies and Walther Vol. 47, No. 3

I i I i 0 400 800 1200 1600 2000 2400 2800

I

LOAD - 25 psi ~

TEMP OF TEST- 2700'F I 10 0 20 40 60 80 100

DURATION OF HOLD (HOUR 1 TEMPE RAT U R E (OF)

Fig. 6. Strength vs. temperature for various brick.

Fig. 8. long-time load test on basic roof brick.

0 I I I

2 3 \! '"OCo>

s @

4 % P I A 4

5 1 LOAD - 25 PSI

HOLD TIME - 5 HR 6 I

2400 2600 -

2800 3000 3200

TEMPERATURE (OF)

Fig. 7. l o a d test of basic roof brick a t various temperatures.

ideal brick for the open hearth would have a wedding of the strength of silica brick and the refractoriness of basic brick. This ideal is now being realized in direct-bonded magnesite- chrome refractories.

One of the interesting properties of the direct-bonded majinesite-chrome brick is the unusual strength-temperature relation. Most refractories have their highest strength at room temperature and show a gradual loss of strength with increasing temperature. Direct-bonded brick form a strength- temperature pattern that is almost in reverse to the normal. Their strength a t 2000°F is about three times the strength at room temperature.

This temperature-strength profile was a t first believed to be caused by a change in mineral phases during heating, but X-ray examination a t various temperatures up to 2500°F, which is beyond the temperature of maximum strength, revealed no ncw mineral phases. It now appears that this strength pat- tern is associated with internal stress caused by the differential cxpansion between the chrome ore and magnesia. I t has been observed in this laboratory in other polyphase systems that contain no magnesia or chrome ore and has been re- ported on in alumina-glass systems by Fulrath.3

The modulus of rupture is a very useful test for rapidly measuring the strength of brick in the low and middle tem- perature range. A very useful test for measuring strength a t

I<. M. Ftrlrath, "Internal Stresses in Model Ceramic Sys- terns," J . Am. Ceram. Soc., 42 [9] 423-29 (1959).

high temperatures is the hot-load test. Hot-load tests can be run by heating the brick a t a scheduled temperature rise under load until they fail or by heating to some desired tem- perature and holding a t that temperature for a prescribed period of time. The brick are then measured for subsidence after cooling.

Initial load tests conducted on direct-bonded brick were of the first type. This procedure has been abandoned, because heating the brick under a load of 25 psi to 3300°P, which is the limit of load-test furnaces presently available, does not cause failure. A sustained load test a t one or more tempera- tures is now used in preference. Figure 7 shows the percentage subsidence in this test on the same kinds of basic brick shown in Fig. 6. These curves are based on holding the brick at the test temperature under a load of 25 psi for 5 hours. The dif- ference between the conventional unburned magnesite- chrome brick and the direct-bonded brick is striking.

Figure 8 shows the subsidence of two brick types a t a constant temperature of 2'700OF as a function of time up to 100 hours. Tests were made in an electric furnace and the subsidence was measured with a kymograph. This test is oiten referred to as a creep test. It shows, as did the previous load tests, the substantial difference in strength between conventional unburned magnesite-chrome brick and the direct-bonded type.

(3) Several factors affect the bonding mechanism, and one of

the more important is the purity of the raw materials. The impurities that most seriously interfere with direct bonding are silica and lime.

Table I shows the results of a study on a series of high-fired magnesite-chrome brick containing about 75Oj, MgO but with a variation in silica content between 1.3 and 3.6%. These brick had about the same appearance and properties at room temperature.

Modulus of rupture a t 2300°F in this series is worth par- ticular attention, as the results of this test have been widely used by both consumers and manufacturers as a measure of direct bonding. The strength at 2300'F gradually increased as the percentage of silica increased. Measurement of the strength of these brick by subjecting them to a load o€ 25 psi a t 3100°F for a maximum period of 5 hours shows silica in a different light, however. When the silica value reached 1.8%, the brick failed during the 5-hour period. At 3.G% silica, the time before failure was reduced to only 5 minutes. The reason for this is readily explained by examining a photomicrograph of the brick with the 3.6% silica (Fig. 9). This brick shows little direct bonding, but the medium-gray area surrounding the chrome grains indicates a high degree of silicate bonding. At the high temperature of the load test the silicate bond gave way and the brick failed.

Role of Purity in Direct Bond Formation

March 1964 Direct Bonding of Basic Brick 119

Table 1. Properties of Burned Magnesite-Chrome Brick with Variable Silica Content

Brick No. 1

Brick No. 2

Brick No. 3

Brick No. 4

Brick No. 5

Chemical analysis (yo) SiO, 1.3 CaO 0 .7

Bulk density (wf) 184

1.6 0 .7 184

1.8 0 . 7 182

3 .0 0 . 8 183

3 .6 0 . 8 182

M O ~ U I U S of rupture (psi)

Load test at 3100"F* 3 8% subsidence 5.0v/, subsidence Failed 285 min. Failed 90 min Failed 5 min.

At room temp. 530 590 610 510 520 i2t 2300'F 1420 1750 1970 1900 2180

* 25 psi, 5-hour (300-minute) hold.

- 3000 n - w

2500 0 0

k W

c 3 (r

0

v) 3 1 500 3 0 0

n

LL 1000

n - 1

Fig. 9. High-fired magnesite-chrome brick showing extensive silicate bonding. Light-gray areas (C) are chrome grains; medium-gray (MI,

magnesia; and darker-gray IS), silicate. (Reflected light, X40).

Tal)le I1 shows the results of a study of the same type of ))rick, except that there is a small variation in the lime con- tent (between 0 7 and 1 8%).

This change in lime contcnt had a small adverse effect on the modulus of rupture at 2:300°F, but its effect on the load test a t 3100°F with a &hour hold was marked When the lime content reached 1 X%, the brick failed before the 5-hour period The results in Table I1 in combination with the re- sults in Table I .;how that, to evaluate a direct-bonded basic brick, the hot modulus of rupture a t 2300°F should be supple- mented with a high-temperature load test. Petrographic and chemical analyses also are most useful.

(4) To those who manufacture basic brick, one of the guiding

principles has always been to make the brick as dense as practical Density or porosity affects a number of the proper- ties of the brick but probably the most important effect i.; on strength A series of high purity, direct-bonded brick was made to determine the magnitude of the effect of porosity.

Role of Porosity in Direct Bond Formation

14 16 18 20 22

APPARENT POROSITY ( % I Fig. 10. Hot strength of direct-bonded magnesia-chrome brick with various

porosities.

Figure 10 shows that the modulus of rupture of these brick a t 2300OF increased in a straight line from 880 to more than 2000 psi as the apparent porosity decreased. The change in porosity not only airects the strength of the brick in the middle temperature range of 2300°F but also has a substantial effect on the strength of these brick a t steelmaking temperature. Figure 11 shows the percentage subsidence of these same sets of brick of variable porosity after heating for 5 hours under a load of 25 psi a t 3100OF. The relation here is no longer straight line but shows the subsidence increasing in an ex- ponential manner as the apparent porosity increases.

Permeability is another property affected by porosity. Figure 12 shows that the permeability of high-purity, direct- bonded magnesite-chrome brick decreased from 1 .G to 0.5 units (a 70% reduction) as the apparent porosity decreased from 20.7 to 17.0%. The value of permeability in basic brick is not well understood and its effect on the service life has not been established. It appears, however, that although the apparent porosity may control the amount of slag or corrosive fume absorbed, the permeability may control the rate of the absorption.

Table 11. Properties of Burned Magnesite-Chrome Brick with Variable Lime Content

Brick Brick Brick No. 1 No. 2 No. 3

Chemical analysis ("/o)

Bulk density (pcf) 184 187 190

A t room temp. 530 520 520 At 2300°F 1490 1490 1150

Load test at 3100"F* 3.8%. 4.1%. Failed

SiOn 1 .3 1 .4 1 . 5 CaO 0 . 7 1.3 1 . 8

Modulus of rupture (psi)

135 min. subsidence subsidence * 25 psi, 5-hour (300-minute) hold.

120 0

lx W

-I

a 4 z

5

Journal of the American Ceramic Society-Davies and Walther Vol. 47, No. 3

6 14 16 16 20 22

APPARENT POROSITY ( % )

Fig. 1 1 . load test of direct-bonded magnesia-chrome brick with various

porosiries.

1.6

> c 1 2 d m a g 0.8

g 0 4

(r W 0.

a

0 15 17 19

APPARENT POROSITY ( % ) Fig. 12. Apparent porosity vs. permeability for

direct-bonded magnesia-chrome brick.

(5) Value of Hot Strength There can be little doubt that direct-bonded brick have

significantly higher hot strength than other basic brick. Strength, however, is important only if it increases the service life of brick. In various service applications, particularly open-hearth roofs, basic brick normally wear away by cracking behind thc hot face and then peeling or flaking. The condi- tions that lead to this type of wear are very complex. When, howcver, cracking occurs, i t can be simply stated that the stress on the brick exceeds its strength. This stress may come from a number of sources including furnace structure, ab- sorption of slag, recrystallization of mineral components, sudden temperature changes, and differential expansion of the chrome ore and magnesite. In the construction of the roof and operation of the furnace some steps can be taken to reduce or to climinate these stresses. Some stresses will always be present, however, and the greater the initial strength of the brick the mure likely it will be to resist these stresses and not crack and peel.

(6) There arc several open-hearth roofs operating in the United

States with direct-bonded brick and a number more are planned,. As of April 1963, however, a campaign has been completed. on only one d,irect-bonded basic brick roof. The campaign was highly satisfactory and has encouraged the be- lief that the exceptional test properties of this brick have not been misleading. The used brick from service have been re- turned to thc laboratory for study and for comparison with other used open-hearth roof brick.

To appreciate what has been learned from this first Ameri- can application of direct-bonded brick, examination of these

Examination of Specimens from Service

v) w e e 0 6 X

0 I 2 3 4 5

DISTANCE FROM HOT FACE (IN.) Fig. 13. Distribution of accessory oxides in used magnesia-

chrome roof brick.

used brick must be set against a background study of hundreds of used conventional magnesite-chrome brick from open- hearth roof service. These studies have typically included petrographic and X-ray examination plus zonal chemical analysis.

Three distinct zones differing in both physical and chemical characteristics are typically observed in conventional basic roof brick from service. The first zone immediately behind the hot face may vary from less than l/z in. to a maximum of about I in. In this zone the brick are subjected to heavy in- filtration of iron oxide and undergo extensive reorganization resulting in a very dense, interlocking, coarsely crystalline structure consisting predominantly of periclase or magnesio- wiistite (MgO-FeO), high-iron spinels, and zoned remnants of the original chrome spinel.

The second zone ordinarily is found I to 3 in. behind the hot face and is characterized by a concentration of both lime and silica, normally in a ratio to form the mineral monticellite. These silicates are derrved from impurities in the brick plus absorption of impurities from the furnace atmosphere and under the driving force of the temperature gradient in the brick have migrated through the intergranular pore structure to the cooler section of the brick to form a dense silicate zone. The third zone starting about 3 in. behind the hot face is unaffected brick.

Chemical analysis of successive segments through the length of a used conventional niagnesite-chrome brick (Fig. 13) clearly defines these zones. Iron oxide is very high a t the immediate hot face and decreases sharply, reaching a normal level a t a depth of approximately 1 in. Lime and silica are relatively low a t the hot face, increasing sharply and reaching a maximum between about 2 in. behind the hot face and then gradually decreasing to the normal level at a depth of about 3 in. The remainder of the brick is essentially unaffected.

The positions of these zones will be altered by anything that alters the temperature gradient. Thus if longer brick are used, or if the brick become insulated by allowing deep dust or kish to settle on the roof, the silicates will move deeper into the brick. Figure 14 shows the s ika t e zonc in a used con- ventional basic roof brick, where the low-melting silicates that form the gray background have virtually engulfed the magnesia and chrome ore. To a major degree these silicates control the physical properties of this zone and i t is in this area that cracks develop.

The used specimens of direct-bonded brick from the first roof trial present some very interesting comparisons and contrasts. The immediate hot-face zone was essentially equivalent to that usually seen in conventional basic roof brick. However, distinct and significant differences were noted just behind the hot face. Figure 15 shows the silicate zone and in this zone the silica content had increased in the normal fashion, but the concentration was only approximately half that observed in conventional refractories.

March 1964 Direct Bonding of Basic Brick 121

ORIGINAL SILICA IN UNUSED BRICK (%) Fig. 16. Silica content of magnesia-chrome roof brick before

and after service.

Fig. 14. Conventional magnesite-chrome brick from open-hearth roof service showing zone of silicate concentration. (Reflected light, X40.)

Fig. 17. Furnace and slag-feeding equipment for simulated chemical zoning tests.

Fig. 15. Direct-bonded magnesite-chrome brick from open-hearth roof service showing zone of silicate concentration. (Reflected light,

X 40.

In this altered zone the direct-bonded brick had the lowest silica content of all the used roof brick analyzed. A careful petrographic examination of the used specimens showed that although extensive penetration and filming by the invading silicates had taken place, direct attachment between the chrome ore and periclase was maintained. The bonding was not dictatcd by the low-melting silicate phases as was in- dicated in conventional compositions. Subsequent stud.ies on direct-bonded open-hearth roof brick later in 1963 have con- firmed these observations.

To place these findings in proper focus, a plot (Fig. 113) was prepared based on zonal analysis of thirty-two separate sets of used magnesite-chrome brick from open-hearth roof service. Brick having six d.ifIerent original silica contents were repre- sented. These tests were mad.e over the past 2 years and include results on both burned and unburned brick. These data show in a statistical way that the silica content in the dcnsificd zone d.ecreased as the silica content of the original brick d.ecreased. Further, the straight-line relation indicates that the silica in the densifjed zone will be about 4 to 5% greater than the silica contcnt of the original brick. Based on a d.iffusing silicate of a composition similar to monticellite, calculations indicate that this 4 to 5% increase in silica in the densified zone would represent enpugh monticellite to virtu- ally fill the intergranular pore structure.

(7) Laboratory Slag Absorption Studies The absorption and penetration of slag and the formation of

the silicate zone is believed to be one of the principal causes of wear in open-hearth roofs. A broad program is under way to study this problem. Results of several tests from this program are presented here, as they involve direct-bonded brick.

To simulate the chemical zoning that occurs in open-hearth roof brick, the furnace and slag-feeding equipment shown in Fig. 17 were used. Test brick are built into a panel facing the burner and the furnace is heated to 3050°F. At periodic intervals granular flush and finishing slag from the open hearth are introduced into the flame. The slag strikes the hot panel, melts, and runs down the faces, a portion diffusing into the brick.

Figure 18 shows the zonal distribution of the combined lime and silica content of two brick types after the slag test. The direct-bonded brick had a significantly lower concentration of lime and silica behind the hot face. Results are in line with zonal analysis of the same type of brick from open-hearth roof service.

Slagged brick from the diffusion test were sectioned into bars (0.8 by 0.8 by 41/2 in.), each representing a 1-in. depth from the hot face. When the hot face of the panel was at 3050'F and the cold face a t 650'F, each one of these bars had some average temperature, which could be estimated from the temperature gradient in the brick. Each bar was reheated to its indicated temperature, and modulus of rupture was determined a t that temperature.

122 Journal of The il merican Ceramic Society -Strickler and Carlson Vol. 47, No. 3

0 1 2 3 4 5

DISTANCE FROM HOT FACE (IN.)

Fig. 18. Distribution of lime plus silica after slogging.

Figure 19 shows the strength profile of the front 5 in. of these brick types containing the absorbed slag The tempera- ture gradient is shown across the top and the distance behind the hot face a t the bottom The strength of the brick became lower toward the hot face as a result of the combination of increa5ing temperaturc and slag absorption. The strength profile developed in these laboratory experiments can well be analogou5 to that developed in brick while in service in the open-hearth roof

Although it cannot bc read directly from this graph, it should also be noted that a t the low strength point, about in. behind the hot face (about 2(i00°F), the direct-bonded

- .- W n - W IK 3 I-

3

U 0 v) 3 -I 3 0 0 z

n a

TEMPERATURE (OF 1 io

" 2 3 4 5 0 1

DISTANCE FROM HOT FACE (IN.) Fig. 19. Strength profile of magnesia-chrome

brick in simulated service

brick was still ten times as strong as the conventional un- burned magnesite-chrome brick.

IV. Summary An up-to-date view of some of the aspects of the direct

bonding of magnesite-chrome refractories is presented. A program of service trials is proceeding rapidly and a number of service campaigns have been completed. Results from these trials indicate there is every reason to believe that the high- fired, high-purity, direct-bonded basic brick will fulfill expec- tations by increasing furnace productivity.

Ionic Conductivity of Cubic Solid Solutions

in the System CaO-Y,O,-ZrO, by D. W. STRICKLER and W. G. CARLSON

Westinghouse Research Laboratories, Pittsburgh, Pennsylvania

The ionic conductivity of cubic solid solutions in the system CaO-Y203-Zr02 was examined. Par- ticular Y203-Zr02 binary compositions were more conductive at elevated temperatures (>6OO0C) than either CaO-Zr02 binary or CaO- YzO3-ZrO2 ternary compositions. The higher ionic conductivity appears to be related to a lower activation energy rather than to the number of oxygen vacancies dictated by composition. Those compositions of highest conductivity lie close to the cubic-monoclinic solid-solution phase bound- ary. Conductivity-temperature data are pre- sented that indicate a reversible order-disorder transition for Yz03-ZrOz cubic solid solutions containing 20 and 25 mole yp YzO3. The trans- ference number for the oxygen ion at 1000°C for Y20s-Zr02 cubic solid solutions is greater than

0.99.

1. Introduction IKCONIUM dioxide has a melting point of -2700°C and transforms from the monoclinic to the tetragonal form at about llOO°C with a large, disruptive volume change.

This undesirable phase transformation can be eliminated by stabilization of the cubic phase with an addition of a selected alkaline-earth or rare-earth oxide. Stabilized Zr02 has been widely utilized in various high-temperature refractory ap- plications. These stabilized ZrOz-base solid solutions also

Z

Presented at the Sixty-Fifth Annual Meeting, The American Ceramic Society, Pittsburgh, Pa., April 30, 1963 (Electronics Division, No. 11-L-63). Received June 10, 1963; revised copy received November 1, 1963.

The writers are, respectively, engineer and supervising engineer, Metallurgical and Ceramic Technology, Westinghouse Research Laboratories