On the stability of refractory materials under industrial vacuum conditions:

CERAMURGIA INTER,NATIONAL,

Vol. 1, n. 2, 1975

8'7

ON THE STABILITY OF REFRACTORY MATERIALS UNDER INDUSTRIAL VACUUM CONDITIONS: AI203, BeO, CaO, Cr203, MgO, SiO2, TiO2 systems D. BERUTO, L. BARCO and G. BELLERI Istituto di Tecnologie e Impianti Meccanici e Istituto di Elettrotecnica, Facolta di Ingegneria, Universita di Genova, Italia

The volatility and stabi'lity of refractory oxides, such as AI203, BeO, CaO, Cr203, MgO, SiO2 and TiO2, towards oxygen has been studied using data of standard free energy for the vaporization processes and the kinetic theory of gases. The analysis has been made removing the congruence condition and examining the system's behaviour in the monovariant region. The results appear to be a useful tool in choosing the experimental strategy for getting simultaneous information on both volatility and stability towards oxygen. Some of the experimental data available in the literature on mixtures of oxides have been compared with ours, and the good qual'itative agreement (~btained will probably allow the application of our approach also to commercial refractories.

1 - INTRODUCTION

The stability under vacuum conditions of refractory materials, such as oxides, carbides, etc., has been the subject of a large number of papers ]-7 mainly concerned with the technological applications in the fields of space research, nuclear energy, and electronic technologies. Owing to the recent extensive development of metallurgical processes under vacuum conditions, this research field has drawn the attention of ceramists and metallurgists who try to obtain stable refractory materials under industrial operative conditions within the limits set by technical and economical conditions, and scientific requirements s, 9, ~o, ]7 This paper deals with the stability towards gases like 02 of pure refractory oxides used in the production of refractory bricks for metallurgical industrial vacuum operations. 2 - THERMODYNAMIC

CONSIDERATIONS

In the absence of liquid metals and of slags, the behaviour of refractory materials under vacuum at high

temperature is substantially the one described by the vaporization processes due to their constituents. For a binary compound of formula A.Bm representing oxides, carbides, etc., vaporization occurs according to the following reactions: A.B~ (c) = A.B~ {g) [1] A.B= (c) = A.B=, {c) + m"B (g) [2] A.B= [c) = A.,Bm, (g) + A.,,B=,, (g) r3] A.Bm [c) = nA (g) -t- mB (g) [4] Generally, the thermodynamic and kinetic study of this class of reactions is carried out through consecutive stages which describe, first of all, the chemical species in the gaseous phase, and the reactions connected with such species. Subsequently, vapour pressure is experimentally determined, and then vaporization kinetics are evaluated. Finally, one tries to work out a detailed model for the whole process which is consistent with the previous experimental remarks. The interest in the first stage of such a study has been considerably increased by the development of high-temperature chemistry 4, 12, 13. At present, at least as far as a large number of pure oxides is concerned. one can determine with sufficient approximation the

88

gaseous chemical species which develop from the vaporization of such oxides 14,15,16. This information is very important both in choosing the vaporization reactions to be studied, and in calculating volatility. Once gaseous species are known, studies of vaporization processes require that vapour pressure should be determined. Usually, experimental measurements of vapour pressure are carried out by Langmuir and Knudsen's techniquelS. 19,~,21 either on pure4 or commer. cial products z3.24. Though such data are a source of most useful information, they must, however, be critically interpreted whenever one wants to use them for materials working under industrial vacuum conditions. In order to further clarify this point, we wish to briefly discuss the underlying postulates on which the determination and calculation of the vapour pressure of a generic binary compound AnBm are based. The vapour pressure of such systems, measured under isothermic conditions, has a reproducible experimental meaning if the degrees of freedom of the vaporization process is equal to zero. The application of the phase rule under constant-temperature conditions provides, for a binary system: v = 3 -- f [5] where: v = degrees of freedom f = number of phases By applying condition5 to the class of reactions describing sublimation processes {eq. [1]}, and to that representative of some types of decomposition, reduction and oxidation reactions [eq. [2]), one can find that such systems always behave like invariants. Application of eq. [5] to the dissociative vaporization processes {eqs. [3] and I-4] gives a degrees of freedom equal to one. Therefore, in such a case, vapour pressure will not have a unique value, but will vary according to the phase composition. For this reason, an additional relationship between the phase composition has been introduced according to the concept of congruent vaporization. This term stands for the particular type of vaporization which allows one to obtain a vapour phase whose composition is equal to that of the condensed phase. Clearly, for reactions described by eqs. 1-3], the relationship is not an equality on a molecular scale, because gaseous species have formulas different from those of condensed components; however, it is true that the numbers of A and B atoms are equal in both phases. Under such conditions, one can write a relationship between the fluxes of evaporating species that is certainly fulfilled when experiments are carried out under high vacuum conditions. Such a condition is verified by laboratory tests using Langmuir and Knudsen's methods, but cannot be true any longer under industrial vacuum conditions. For instance, the presence of the liquid metal bath reacting with oxygen causes, as a final effect, the partial oxygen pressure in the atmosphere of the industrial plant to be changed. If we add to these reasons also the influence of temperature fluctuations and gas composition on the partial oxygen pressure in the environment during usual practice, we can easily conclude that, under industrial vacuum conditions, it is more realistic to consider an evaporating refractory system as bivariant, rather than invariant. Therefore, under industrial conditions, volat i l i t y will be a function of the composition of the gaseous phases, and at equilibrium will vary as they do according to the law of mass action. Data obtained on pure ceramic oxides by some researchers 27 show the dependence of volatility on the nature of materials and on environment conditions, and, according to this work, can be interpreted, as far as the influence of environment conditions is concerned, as the vaporization of systems that are not at the congruence point, in the following, we shall develop some

D. BERUTO, L. BARCO, G. BELLERI

remarks made by Kellog ~ in the field of extractive metallurgy, and shall also present the monovariant behaviour of the dissociative isothermal processes relevant to the systems: MgO, CaO, AIzO 3, TiO2, SiO z, Cr203, and BeO. This approach will allow us to pursue Searcy's ~ suggestions on the calculation of the maximum vaporization rate for the metallic species of a monovariant isothermal system as function of oxygen. Clearly, a gas-solid rate reaction cannot be higher than the rate at which the gaseous reagents can reach the solid reactant surface. Once the partial pressure of the reagent gas is known, such a rate can be easily calculated by Hertz- Langmuir ~ equation: J~. . . .

=

[2~ M~ RT}-~/2 Pi,eq

[6]

where Ji.max is the flux of the i-th component expressed in (moles/cm 2 sec} Pi.eq is the equilibrium pressure of the i-th component M~ is the molecular weight R is the constant of the gases, and T is the absolute temperature. Under thermodynamic equilibrium conditions, eq. [6] yields also the maximum rate at which the gaseous products leave the reactant surface 6. Since, through the law of mass action, it is possible to know the partial gas pressure of the metal species as a function of that of oxygen, it follows that also its flux, according to eq. [6] will be a known function of the partial oxygen pressure. It is worth noting that, generally, under industrial vacuum conditions, such a value does not coincide with that determined by the congruence condition; therefore, the maximum vaporization rates calculated for monovariant systems will differ from those tabulated in the literature. In our results, these will be represented by a point, and any variation from the partial oxygen pressure reached at the congruence point will make the evaporating flux displace according to the law determining the monovariance of the system, in addition, the slope of such curves, which is a function of the various chemical equilibria occurring during the vaporization process, provides a measurement of the sensitivity of the evaporating fluxes towards the species of oxygen. Indeed, the variation of volatility P----PM +Po2, as a function of the partial oxygen pressure, can be written as: (~P/SPo~) = (aPM/aPo2) + 1, and therefore one can say that the variations of the evaporating fluxes of the metallic species, as functions of the partial oxygen pressure under industrial operative conditions, stem from the evolution of the oxide volatility. If we assume this evolution as a criterion for the oxide stability towards oxygen, it follows that the slope of the plots that we shall illustrate in the following will be reflecting the relative stability of the different refractory oxides. 3 - SYSTEMS MgO AND CaO

A detailed discussion of these vaporization processes is already available in the literature 14, is. 31,32.33; so, in the following, we shall consider only reaction of the types: a b

MgO(c) = Mg(g) 4- 1/2 O2(g) CaO[c) = Ca[g) + 1/2 O2(g)

The application of the law of mass action to the monovariant system a}, under the assumption that the activity of the phase MgO (c) is equal to one, yields: exp (--AG°/RT] = PMgPo2 '/2

LT]

A G° being the standard free energy change connected

ON THE STABILITY OF REFRACTORY MATERIALS UNDER INDUSTRIAL VACUUM CONDITIONS: AI203, BEE), CaO, Cr203, MgO, SiOz, Ti,O2 systems

with reaction a). It is an easy matter to obtain from eq. [7] the expression of the flux of Mg as function of the partial oxygen pressure. In fact, in accordance with eq. [6], the flux of Mg can be written as follows JM~ ----- [2~MM+RT]-~/~ P~+

[8]

By applying eq. [7] to eq. [8], and by expressing the result in gin. of MgO/cm2h, one can obtain: JM,o -: (1596. 105/T'/2) M~o[exp(--AG°/RT]/Mu~ "2] 1/Po2'/~ [9] where: JMgo is the maximum flux of Mg expressed in gm of MgO(gm/cm2h) Po2 is the pressure of O2 expressed in atmospheres MM~o and MM~ are the molecular weights T is the absolute temperature. Therefore, the maximum flux of the metallic element, at a fixed temperature, varies as a function of Po2, in accordance with eq. [9]. In case the vaporization of the magnesium oxide should take place under congruent conditions, one can establish the following relationship between the fluxes of M,, and O~. J~ : 2J,~2(moles/cm 2sec)

[10]

Such a relation, expressed in terms of pressure according to eq. [6] and combined with the law of mass action yields: Po, - E2 exp(AG"/RT)(M~/Mo3 '2] .~3

[11]

With this value of the partial oxygen pressure, the evaporating flux described by eq. [9] will assume the maximum possible value (Jc]. Fig. 1 shows the evaporating fluxes for the processes a] and b) at the 1600 °K and 1900 °K temperatures.

B

. . .

89

According to Searcy29 the maximum rate at which the MgO vaporizes would still be Jc. In such a cases eq. [9] does not accurately represent JM~o, and the quantity Jc remains a more accurate value for this maximum rate. In this region eq. [9] gives simply the flux of Mg expressed in gm of MgO. This flux can became even larger than one calculated at the congruence point. By using the data provided by Majdic35 for MgO in the presence of liquid steel, one can find that the flux of Mg shifts to the value of 4x10 ) gm/ cm 2 h compared with that of 1.9x10 2 gm/ cm2h at the congruence point. In Fig. 1, such a flux is denoted by JM. Obviously these considerations apply to all the refractories considered in this paper. For CaO the data relevant to free energy have been provided by Coughlin36 The highest limit of the fluxes in Fig. 1 is due to the fact that, once the temperature is fixed, a decrease in the pressure of O2 causes a rise of the metal pressure, and such a rise will last till the saturation pressure of the metal is reached. Then, the precipitation of the liquid metal will begin, and the horizontal lines will represent the invariant system: Mg [c) _-- Mg (g). The initial points of such areas have been calculated under the assumption that the oxygen solubility in the liquid metal is negligible; for this reason, they are not to be considered very exact. However, they are meaningful as far as the areas delimiting the stability of the condensed species MgO (c) or CaO [c) from that of the phases Mg (c) or Ca [c) is concerned.

4 - SYSTEM

Cr-O

The vapour pressure of the species in equilibrium with CQO 3 (c) has been measured by Grimlej et alY. As Brewer pointed out 33, a good agreement between the experimental and the calculated value af vapour pressure can be obtained if one assumes the species CrO [g) to be more important than the gaseous metal Cr (g). Recent studies 4°.41 on the vaporization of the system Cr2O3 (c) in oxidative environments have pointed out the existence of other gaseous phases. However, as far as our purposes are concerned, we have restricted our calculation only to the reaction: Cr203(c) -- 2CrO(g) + 1/2 O2(g) and we have expressed the flux of CrO i~ gm of Cr203/cm2h as:

I-JB

MgO(c),CmOe) s t a b l e b e l o w p o i n t s A,B Mg(c) Ca(el s t m b l e a b o v e p o i n t s A,B

• JO

• ']'C

~;

-30

25

20

-1'5 Log Po2

FIGURE 1 maximum n~tal as MgO and

J~:,2o~ = [1.596. 10~/T"-~)M,:,2m[exp (--AG'/RT)]"2/Po2~'2 • Mc,o'/2 [grn/cm 2h]

F

• 'I'M

-10

-5

[Atm]

- Logarithmic diagram relevant to the variation of the evaporating flux of the gaseous species containing the function of the partial oxygen pressure for systems Ca-o.

It should be noted that equation [9] well defines the maximum flux of MgO under congruently vaporizing conditions. For other conditiones the same equation does not necessarly represent JM~o- However, since MgO is the only source of Mg atoms whereas the 02 is present from other sources, we think that eq. [9] is still useful for obtaining kinetic data in the region where the pressure of 0 2 is increased above the value corresponding to Jc given by eq. [11]. According to this, if there is more oxygen in the environment, as happen under industrial vacuum conditions, the flux decreases from the value Jc to that relevant to the new partial pressure of 02 in equilibrium with the metallic species. Where Po2 is smaller than the value given by eq. [11] it is necessary to distinguish between J~, and J~,o.

[12]

The evaporating flux relevant to the region where the condensed phase Cr Cc) is stable has been calculated from the reaction: Cr(c) + 1/2 Oz = CrO(g] on the basis of the data provided by Brewer 33, whereas the procedure followed to calculate the congruence points is quite analogous to that illustrated for the species MgO and CaO.

5 - SYSTEMS

AI-O

The studies carried out by De Maria, Drowart et al. 39 allow today's user to know the gaseous species associated with the vaporization of alumina. Table I gives the reactions here considered both for the region where the condensed phase AI:O 3 is stable, and for the region where AI (c) is stable. The reactions have been studied using the data provided by JANAF Tables 34 at various temperatures, and in Fig. 2 are shown the results relevant to 1900 °K and

90

D. BERUTO, L. BARCO, G. BELLERI

TABLE I - Reaction describing the vaporization process for system AI-O. Condensed Phases

Equilibria

AI~O~ (c}

AloOf(c] = Al=O(g) + O~(g) (a) AlcOa(c) = 2 AIO(g) + 1/20~(g) (b) AI,O~(c) = 2 Al(g) + 3/20~(g) (c)

AI (c)

2 Al(c) + 1/20=(g] = AlsO(g) Al(c) + 1/20~(g) = AIO (g) Al[c) = AI (g)

T -- 1900'K

--

T_-1600°K

. . . . . .

A

(d) (e} (f)

A,=O~Co).=.b,e be,ow po~.=. A

, IA

(0

A'(c) .=able

above

pointe

0

A

t,

The application of the additional congruence condition yields, in a manner quite analogous to the previously indicated one and for the overall flux: J,]/Jo = 2/3 (moles/cm2sec}

As the gaseous phase consist of AhO(g), Al[g), AlO(g) and 02(g), such a relation becomes: 4/3 J,~2o+ 1/3 J,,o + J,, = 4/3 Jo2[moles/cm2 sec)

'±215 '

-2L0

-1]5= ' ' -~1;0 ' [A',m]

"J5'

Log Po2

FIGURE 2 - Logarithmic diagram relevant to the variation of the partial pressure of the gaseous species conteining metal AI as function of the partial oxygen pressure.

A

Ai203

[15]

The solution of the equations system made up of expression [15] together with the laws of mass action relevant to the reactions of Table I provides the value of the partial oxygen pressure to which the evaporating flux, denoted by point Jc in Fig.. 3, corresponds. Deviations from the congruence point J¢ should occur under industrial conditions and, in the case of poor vacuum, we should expect a greater stability towards oxygen. 6 - SYSTEM

L310'

[14]

Si-O

The gaseous molecules developing from the vaporization of SiO, are those of monoxide SiO and of SiO2(g) ~4. Therefore, on the ground of the tabulated data relevant to the standard free energies of the two compounds, it is possible to study the reaction shown in Table II following a procedure analogous to the previous one. In Fig. 4 are shown some results obtained for reactions a), b), c], d) at 1600 °K and 1900 °K temperatures. In this case, too, we have calculated the total flux of Si, expressed in SiO2, using the formula: Js,o2= (1.596 - lO~/T]/2)Ms,o2(P~o2/Ms~o2'/2+Ps,o/Ms,o '/2) [gm/cm2sec] [16]

1 5

The graphs relevant to such a formula; rewritten as function of Po2, are illustrated 'in Fig. 6 and will be discussed in the following. We want to note here that, as reaction a) is invariant, the calculation of the congruence point in the SiOz(a} region has been carried out taking into account only reaction b), that is: AI203(¢ ) e t a b l e AI()

.tmbl..boY.

. . . . .

"~. ~~-10

b e l o w pol

2'5 '

1

p ......

'

:

-1'5 . . . .

L2'0 . . . . Los

-1'0 . . . .

-5'

.

Js,o = 2 Jo2 (moles/cm2sec)

[17]

~ o~

TABLE II - Reaction describing the vaporization process for system Si-O.. Condensed Phases

Equilibria

SiO~(.c}

SiO~(c) = SiO2(g) (a) SiO2(c) = SiO(g) + 1/2 O2{g) (b)

r0

PO2 [Atm]

FIGURE 3 - Logarithmic diagram relevant to the variation of the maximum evaporating flux of the gaseous species containing a metal as function of the partial oxygen pressure for system AI.O.

Si(c) + 1/20~(g} = SiO(g} Si(c) -t- O~ (g) = SiOz [g)

Si(c}

1600 °K temperatures. By writing the total flux of AI in gm of AhO3/cm2h as:

i

J,,2o~ = (1.596. 105/T'/2) MAI2o~(1/2PA,~g)/M,,'~ + 1/2 PA,o/M,,o'/2 + P,,2o/M,12o'"}[gm/cm 2h] [13] and by substitution in eq. [13] of the partial pressures of the species containing the metal with the equilibrium constants relevant to eqs. a), b), c], we obtain the relationship linking the total flux to the partial oxygen pressure. Fig. 3 represents such a function at 1900 °k, 1800 °K, and 1600 °K temperatures. All these curves bring into evidence the existence of two regions where the stability towards oxygen is different. This result is consistent with the functions shown in Fig. 2. In fact, the range of the partial pressures of Oz where a greater stability of the evaporating flux is noted partly coincident with that where the production of the gaseous species least sensitive to the oxygen variations (i.e., A10(g)) is energetically favoured with respect to the other species.

J

J

/

/

.

,

30

,

,

r

.

,

,

,

,

-

,

'

I

Sb02(91~

19oo'k (bF'~

SJoz(g)

,.8oo~..

8102(©) a r a b i a

b e l o w polntm A above points A

el(c) s t a b l e

(c) (d)

' -1'5 ~ ' ' "1'0 . . . .

Log Po2 IAtm]

-'5

T 10=_=

15

'

'

'

'

FIGURE 4 - Logarithmic diagram relevant to the variation of the partial pressure of the gaseous species containing Si as function of the partial oxygen pressure.

ON THE STABILITYOF REFRACTORYMATERIALSUNDER INDUSTRIALVACUUM CONDITIONS: Al203, BeO, CaO, Cr203, MgO, SiOz, TiOz systems

7 - SYSTEM

91

Ti-O (c)

Be

Ig)

The system TiO2]~, which, in a sense, presents a vapour phase homologous to that of SiO2, differs from this case owing to the stable molecules in the condensed phase. Then, following the information provided by JANAF Tables 34, we have studied the vaporization processes shown in Table III.

T 1900 'K :

" ~

' --5

III - Reactions desoribing the vaporization process for system Ti-O.

TABLE

Condensed Phases

Equilibria

TiO2(c)

TiO2(c] = TiO2[g] TiO~[c] ---- TiO(g] 4- 1/2 O2[g)

[a] (b)

TiO[c]

TiO[c] = TiO(g] TiO(c} 4- 1/2 O2(g] = TiO2[g]

(c] [d]

The temperature range in which such processes have been examined is still the one between 1600 °K and 1900oK. The total flux of the evaporating metallic species and the congruence condition have been studied on the basis of the following equations: J~,o, = [1.596 - 10'/T ''~] M[~o2(PT,<>2i~>/MTio21124- P[io<,,/M~
8 - SYSTEM

Be-O

The vaporization of the compound BeO, compared with the previous cases, seem unique owing to the presence of polymeric species whose effect is considerably important as far as the stability of BeO towards O 2 is concerned. The reactions we have studied using the thermodynamic data provided by JANAF Tables are those shown in Table IV. IV - Reactions describing the vaporization process for system Be-O. TABLE

Condensed Phases BeO(c) Be(c]

All

cases

Equilibria BeO[c] = Be(g) + BeO[c) = BeO[g)

1/20~[g)

Be[a] = Be[g) Be(c) + 1/2 O2[g) = 2 3 4 5

BeO[g) BeO(g) BeO(g] BeO[g]

= (BeO)2 (g) = (BeO]3 [g) = (BeO]~ (g] = [BeO]5 (g]

BeO(g)

[a) [b) (c) (g] (e] (f) [g) (h)

I\¢\•

%~ / ~ 0~-~' ~"

B e (c) s t ; a b l e

above

A

points

~, ~,.~ 40

-20

-25

-20

-1'5~-T-~0 Log % EA,mi

-

~ - ~ - -

0

FIGURE 5 - Logarithmic diagram relevant to the variation of the partial pressure of the gaseous species containing Be as function of the partial oxygen pressure.

whereas the congruence points have been determined in accordance with the equation: [21]

JB. : 2 J(~z [moles/cm~ sec)

9 - DISCUSSION In this section, we want to briefly re-examine and compare the results obtained previously in order to discuss the information provided by the study of monovariant dissociative vaporization processes. To this end, we choose the 1900 °K temperature as a reference, since it is rather similar to those relevant to the treatment of steel under vacuum conditions. In Fig. 6 we have represented the functions relating the fluxes of the metallic species expressed in gm of oxide {gm/cm2h) to the partial oxygen pressure in equilibrium with the system that is undergoing the vaporization process. The criterion we shall use to interpret the graphs of Fig. 6 is based on two principles: the first one concerns the amount of evaporating flux, whereas the second is relevant to the stability of this quantity towards the partial oxygen pressure. We begin by noting that, as far as the first parameter is concerned, adequate information on the evaporating species under congruence conditions leads us to a series of values constituting the maximum evaporating flux under the operative conditions obtained in a laboratory by applyMr~O

- --~

T=1900 °k

~.

[1

""

5

" Jc

:10

l~20~ B~0

The equilibria represented by the equations of Table IV have been studied in the 1600 °K-1900 °K temperature range, and the results relevant to 1900°K temperature are shown in Fig. 5. (BeO)3 seems to be the most abundant among the polymeric species, and the set of such molecules, beginning from a partial pressure of O2 equal to 10 6 atm, is definitely favoured compared with the metal Be[g). Due to the invariance of the equilibrium from which polymeric species originate, one can easily realize that, in such a region, the flux will have to be stabler towards oxygen. The results prove this forecast, a'nd the total flux has been calculated according to the formula: J.,.o - (1.596. 10~/T'~] MB~o[P.,./MB~'2 + P~o/MB~o''z + 2 P..2off M.,,,o~''2 + 3P.~.,o,/M,~.~o~'~ + 4PB~,offMB~.~o,'~ + 5P~,o~/M.~o~''~) [gm/cm 2 h] [20]

C,:~.--''" ~

• 30

25

Jo -20

-1'5

-10

-5

0

Log Po2 iA'mJ FIGURE 6 - Logarithmic diagram relevant to the variation of the maximum evaporating flux of the gaseous species containing a metal as function of the partial oxygen pressure for the systems: AI-O, Be-O, Ca-O, Cr-O, Mg-O, Si-O, Ti-O, and Zr-O.

92

D. BERUTO, L. BARCO, G. BELLERI

ing Langmuir and Knudsen's techniques. Even if such data cannot de d i r e c t l y extrapolated to the industrial vacuum conditions, they can provide, however, valid i n f o r m a t i o n for the calculation of the v o l a t i l i t y of pure oxides. Nevertheless, they do not yield by t h e m s e l v e s any information on the stability of the same materials t o w a r d s the oxygen gaseous phase. In fact, to obtain such information, one needs to know the function linking the evaporating fluxes to the v a r i a b l e Po=. In the f o l l o w i n g , using the f o r m u l a s discussed i,n the previous section, w e have calculated, for the various condensed phases, both the amount of the evaporating fluxes, and the d e r i v a t i v e of these function to the congruence point and to the point, called , w o r k i n g point ,,, w h i c h is relevant to the conditions that could be obtained under industrial vacuum conditions. The results of such a calculation are shown in Table V. As far as the s t a b i l i t y of the various oxides under consideration is concerned, one can note that, at the c~ngruence point, SiO2 MgO, Cr~O~, CaO, Tie2, and B e e present an analogous behaviour, w h e r e a s AI20 3 turns out to be the most unstable species t o w a r d s oxygen. V - Comparison between the maximum evaporating fluxes a~d the . stabilities. (dJ/dPoD of the condensed phases: AI20~, Bee, CaO, Cr=O~, MgO, SiO2, TIe2, and ZrO2 calculated at the congruence points (Jc), and those calculated at the point (Jo} relevant to a tentative behaviour, under industrial vacuum conditions, in the absence of a metallic bath. TABLE

Conddensed Phases SiO2 MgO Cr203 CaO Tie2 Bee ZrO2 AI20~

Qualitative comparison between the volatilities of basic and high-alumina commercial bricks and the one which can be calculated from pure oxides in accordance with the present article.

TABLE Vl -

High Alumina Brick

Basic Brick

Measured increasing stability of the volatile components. Ref. {22}

Calculated data on pure single oxides, This work.

Measured increasing stability of the volatile

SiO2

SiO2

SiO2

SiO2

Tie2

Tie2

MgO

MgO

AI203

AI20~

Cr203

Cr203

components. Ref. (22}

Calculated data on pure single oxides. This work.

resulting from our research. Such data have been shown in Table VI, w h e r e the various species have been tabulated in a decreasing order of volatility. However, it should be stressed that the transition from pure oxides to m i x t u r e s of these gives rise to chemical bonds and other kinetic parameters w h i c h g e n e r a l l y alter the vaporization process.

ACKNOWLEDGEMENTS

The discussion held with Prof. G. Aliprandi have been very useful in the initial stage of this research work. Dr. A. Scavotti has helped in elaborating data.

Jc [g/cm 2 h) - (dJ/dPo=} c Jo (g/cm 2 h} - (d J/alP02) o 5.7x 10~ 1.9 x 10-5 1.8x 10-2 7.9 x 10-~ 3.0x 10-s 1.4 x 10 -s 1.2 x 10-~ 1.3x 10-~

3.9x 10 4 2.6 x 10 4 1.7x10 4 3.6 x 10 ~ 5.2x 10 4 1.6 x 10 4 O 1.1x10 ~

1.3x 10~ 3.1 x 10~ 3.5x 10-~ 1.8 x 10-5 8.9x 10-~ 2.3 x 10-~ 1.2 x 10-~ 4.8x 10-~

1.6x 10 2 3.5 x 10 5.4 2.5 x 10-j 6.1 x 10-2 1.0 x 10-3 O 4.0 x 10~

If this behaviour is important, one can easily understand w h y ZrO2 is an e x c e l l e n t oxide; in fact, this condensed phase presents a l o w evaporating flux, and the e q u i l i b r i u m describi~ng its vaporization is invariant t o w a r d s oxygen. The results obtained under industrial vacuum conditions, w h e r e w e assumed a P02 w h o s e value m i g h t range b e t w e e n 10 -4 and 10 -5 atm, p o i n t out that the s t a b i l i t y of the oxides g e n e r a l l y increases as the partial oxygen pressure increases. However, the rise of this parameter turns out to be considerably s e l e c t i v e as far as the various species are concerned; a more d e t a i l e d analysis seems to provide meaningful hints. In fact, w h i l e AI203 was p r e v i o u s l y the most unstable species, now it appears to be the stablest, and Bee, as it increases its s t a b i l i t y by a 10~ factor, becomes a d e f i n i t e l y interesting material. Besides, the increase in s t a b i l i t y undergone by CaO is greater than that of MgO by a 102 factor, and the phase SiO2, though it increases its s t a b i l i t y by a 102 factor w i t h respect to the congruence point, is still the m o s t v o l a t i l e and least stable oxide. Finally Tie2, under industrial vacuum conditions, turns out to be a material which, even if it does not reach the values of AI203 and Bee, should behave w i t h i n suitable limits as regards both the evaporating flux and stability. In conclusion, it seems meaningful to note h o w the data obtained by Bonar n ~n the field of c o m m e r c i a l refractories present a type of v o l a t i l i t y that, from a q u a l i t a t i v e point of v i e w , is consistent w i t h the data

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ON THE STABILITY OF REFRACTORY MATERIALS UNDER INDUSTRIAL VACUUM CONDITIONS: AI203, 8eO, CaO, Cr203, MgO, SiO2, TiO2 systems

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93

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Received January 26, 1975; revi,sed copy received May 30, 1975.