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Behavior of Refractory Oxides in Contact with igh Temperatures GEORGE ECONOMOS Massachusetts Institute of Technology, Cambridge 39, Mass.
M
ODERN high temperature requirements of nuclear energy and supersonic flight have brought new and more rigid demands on known refractory materials. Quite often it is necessary to use metals in contact with refractories at these elevated temperatures; therefore, i t is essential t h a t their behavior under these conditions be known. This qualitative study of a number of these combinations was made as a prelude t o a more extensive examination of those combinations which might find useful application. Apparatus and Procedure The apparatus used in attaining the desired firing conditions consisted of four major units as shown schematically in Figure 1. The vacuum system was standard equipment with a fore pump and oil diffusion pump. The induction generator was a lOO-hp., 50-kw., 9600-cycle Tocco unit. The induction furnace heating unit was a molybdenum assembly within a gas-tight, fused silica tube and was inductively heated by the induction coil surrounding it. These units have been described in detail in earlier publications b y the designer (9). The final unit, the furnace gas purifier, was a train consisting of a cold trap of an alcohol-dry ice mixture to remove water vapor, a chamber containing magnesium chips heated a t 700" C. to remove oxygen, and more dryers of calcium sulfate and phosphorus pentoxide in series to remove any residual moisture from the gases being introduced into the furnace. The refractory oxides chosen for these experiments were those commercially available, yet were of high purity. All the oxides % in used (AlzOa, MgO, BeO, ZrOz, TiOs, ThOs), were 99 purity, except for the ZrOz which was stabilized with about 4 % CaO and contained about 4% HfOz. The oxides were fabricated into reaction disks 0.76 inch in diameter and 0.25 inch thick, and cover disks having 0.125-inch recess. These were sintered, and then the contact surface was given a high polish to minimize purely mechanical adherence t o the metal under test. I n addition, TiOz was reheated in vacuo to 1600" C. giving it a black color. (X-ray examination showed only rutile present.) The metals titanium, beryllium, silicon, niobium, nickel, molybdenum, and zirconium, either pellet or powder of 99 -/- yo purity, were placed on the flat, polished reactor disks and each assembly was enclosed by a cover disk. These were placed into the induction furnace susceptor and fired in an inert atmosphere to 1400°,l6OO0, and 1800O C . These were furnace temperatures obtained by sighting on the specimens. Each firing contained one oxide with all the metals. This was deemed advisable t o avoid the violent reaction occurring when the different oxides made contact. A rather complex firing schedule was employed. After loading the specimens into the furnace assembly, the system was evacuated overnight to 0.05 micron of mercury. Purified hydrogen (30om. of mercury) was admitted, the furnace was heated t o 600' C., and then the system was evacuated to 0.5 microns. (This was an added precaution to remove residual oxygen.) Purified helium (30 cm. of mercury) was next introduced and the
desired temperature was attained a t a rate of 20' C. per minute. This temperature was held for 15 minutes, then the power waB shut off and the unit was permitted to cool before opening. The reacted specimens were next examined visually to ascertain the physical changes, such as corrosion, oxidation, adherence, volatilization, and discoloration. Metallographic and petrographic examinations of polished sections and thin sections, respectively, were used to show any alterations of the oxide phase. To obtain something more concrete about the reaction products, loose mixtures of the metal and the oxide were fired a t 1800" 6. in crucibles of the oxide under consideration. The mixtures were then subjected to x-ray analyses to identify the products formed. ExperimentaI Results and Discussion The results of these tests together with any reaction products identified have been summarized in chart form in Table I. Lower-melting silicon and beryllium gave excessive volatilization a t the highest test temperature (1800' C.) They were, therefore, omitted when firing at this temperature because of the excessive damage caused when condensation occurred on the cooler parts of the molybdenum furnace. Other omissions were as noted on the chart. The oxides studied are generally regarded as stable one8. However, when reaction between oxide and test metal occurs, it becomes more pronounced with temperature increase. At 1400" C. all combinations tested n-ere found to show relatively low reactivity. Titanium showed a bit of discoloration on magnesia, and niobium showed some discoloration on stabilized zirconia. Beryllium and silicon showed early reactivity with a number of oxides, beryllium with magnesia and titania, and silicon n ith alumina, magnesia, and titania. At 1600" C., the reactivity of those given a t 1400" C. was fo.und to have increased. I n addition, titanium attacked all the oxides except thoria, and niobium attacked titania and to some extent,
+
FURNACE
DIFFUSION PUMP Figure
458
1.
FORE PUMP
Induction H e a t i n g Apparatus
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
February 1953
Table 1.
AlzOi RPO _. .
MgO ThOz TiOa ZrOz
Temp., 1400 1600 NR NR * NR NR NR NR NR NR NR NR
*
1400 AlzOa BfB MgO ThOz Ti02 ZrOz
Mo C. New 1800 products NR ... NR ... NR ... NR
...
... ...
**
NR Be
C. 1600 1800
Temp.,
- - NR
NR SR VR
SR
Reactioh Summary Chart"
Ni Temp., C. New 1400 1600 1800 products NR NR NR * ... ,-. NR NR. . . _ NR NR NR ** NR N R
* *
~ New products BeO*AlzOa
Nb Temp., C. 1400 1600 1800 NR NR NR SR VR NR NR SR NR SR NR SR ** SR SR VR
...
* .'E
459
* *
... . .. ...
Ti Temp., O C. 1400 1600 1800 NR SR SR VR SR SR ,E * NR NR NR SR ** NR SR L R
New products NbzOs NbtOs . . NbaOs
E
*
... . .. ...
Si
-__ Temp., 1400
y
1600 VR VR VR VR VR -VR ,
C.
-
1800
0
New products Ti02 TiOs MgO*TiOi TiOa 7
TiO; S.S. of T i
Zr New products
SiOz: 3Ala0~2SiOa
Temp., O C. 1400 1600
NR
1800 VR VR SR NR ._.
-
New products ZrOz
SR z102: (7) SR SR 0 Be0 0 NR ZrOz SR * * ZrOa SR ... NR ** 1 SiOn NR SR ** 7 SR VR SR NR S A 0 Be0 0 ZrOn-SiOa NR SR ... NR NR S R = slight reaction as shown by surface discoloiation and penetration: VR = violent reaction a s shown by a Code: N R = no peiceptible reaction; omitted because of excessive volatilization of the metal at 1800" C * = omitted because Ti02 fused presence of a new phase or surface corrosion; 0 = omitted because only few specimens were available (strategic materials): 7 = x-ray lines show ididentified reaction product: 6 . 8 . = helow 1800° C.: solid solution shown .by d-line shift i n x-ray study. b Underscoring indicates molten metal in contact with the oxide.
-
- .-
Y
-
*
'
...
beryllia. Beryllium and silicon also began t o attack the remaining oxides. For the combinations tested at 1800" C., reactivity appeared to be widespread. Even zirconium showed attack on zirconia and magnesia. Thoria was the most stable oxide at this temperature, although even i t showed reactivity with niobium in addition t o the reactions with beryllium and silicon mentioned earlier. The only two metals which showed no apparent effect on all the oxides were nickel (11, 12) and molybdenum (2) which substantiated work reported earlier. For the other metals, some reasonable explanations can be offered for their reactivity. Beryllium, being the most reactive of the metals used (8, IO), began to attack early, reacting also with thoria and to some extent even beryllia. I n these cases where no new valence states have been definitely established, solid solution seems to be the only explanation. However, further study is needed to clarify this point. Niobium showed most of its reactivity at the more elevated temperatures, and here also solid solution appeared t o be the best explanation. Titanium (1, 4 ) and zirconium (3) are metals which showed extremely complex behavior. They have been reported as being able to take oxygen into solid solution up to as much as 50 atomic % (6). This offers an excellent mechanism for oxygen interchange which can permit great reactivity. Silicon also showed peculiar behavior (7, 1 3 ) . Even a t the lower temperatures, reaction was noticed and often the metal disappeared completely. The only reasonable explanation was the possible formation of the suboxide (6, 14) or the silicides. (None of these was identified in the x-ray study.) X-ray examination of the loose oxide and metal mixtures was not very enlightening. In some cases where no reaction was observed, a new oxide was identified, and where a reaction was observed, no new compound was found. The poor degassing properties of these loose powder mixtures could be an explanation t o the former difficulty. The latter deviation could be that a small amount of reaction product would be difficult to identify b y x-ray techniques. Extensive study by use of radioactive tracer techniques would aid in clarifying these questionable issues. Conclusions At temperatures below 1400O C. in an inert atmosphere,. the oxides A1208, ZrOz, BeO, MgO, TiOz! and ThOz, were esse?tially unaffected by the metals Be, Zr, Ti, Nb, Mo, Ni, and Si. At
*
* **
... 2MgO.SiOs ...
* NR *
-
3
**
more elevated temperatures, 1600' and 1800° C., reactions between these combinations became pronounced. The relative order of decreasing stability of these oxides was found to be Thon, ZrOz, AlzOa, BeO, MgO, TiOz. Molybdenum and nickel were the only metals of those tested which were stable in contact with the oxides at all temperatures. Beryllium and silicon showed early reactivity with some of the oxides and attacked all the oxides at the more elevated temperatures. The former metal is known for its high reactivity, while the latter metal could possibly form the suboxide or the silicides from the oxides. Zirconium and titanium with their high oxygen solubility reacted with the oxides, probably b y some sort of oxygen exchange mechanism. Their reactivitv increases with temDerature increase. Niobium showed reactivity mostly a t more elevated temperatures. This and a few other reactions could be explained as solid solutions at these elevated temperatures, but further investigation is needed to obtain more conclusive proof. Acknowledgment Acknowledgment is made t o the sponsor of the project under which this work was done: NEPA Division of the Fairchild Engine and Airplane Corp. The author also wishes t o thank F. H. Norton and W. D. Kingery of the Ceramics Division of Massachusetts Institute of Technology for their guidance in this investigation. literature Cited (1) Brace, P. H., J. Electrochem. S O C ,94, 170 (1948); IND.ENG. CHEM.,42, 227 (1950). (2) Cronin, L. J., A m . Ceram. SOC.Bull., 30, 234 (1951). (3) Cubicciotti, D. D., J. Am. Chem. SOC.,7 3 , 2032 (1951). ~ .albem. Chem., 247, 53 (1941). (4) Ehrlich, P., 2.U ~ O T u. (5) Erasmus, De With, and Persson, J. A., J. Electrochem. SOC.,95,
j
316 (1945). (6) Fast, J. D., Metatlwirtschaft, 17, 641 (1938). (7) Gire, O., Compt. rend., 194, 884 (1932). (8) Jacobson, C. A., "Encyclopedia of Chemical Reactions," hTem York, Reinhold Publishing Corp., 1949. (9) Johnson, P. D., J. Am. Ceram. SOC.,32, 316-19 (1949); 33, 168 (1950). (IO) Kura, J. G., et aE., J . Metals, 1 , No. 10, Trans. 769 (1949). (11) McMahon, J. F., et al., Office of Naval Research, Contract N6-Ori-143-TO-2, Summary R e p t . (July 22, 1949). (12) Navias, L., J. Am. Ceram. SOC.,19, 1 (1936). (13) Pidgeon, L. M., and King, J. A., Discussions Faradall SOC.,4, 197 (1948). (14) Wartenberg, H. V., 2. Elektrochem., 53, 343 (1949). RECEIVED for review M a y 20, 1952. ACCEPTED September 22, 1952. This work was taken from a thesis presented by George Economos in partial fulfillment of the requirements for the degree of master of science in ceramics a t Massachusetts Institute of Technology, June 1951.
