PREPARATION BY THE OF

TECHNICAL

CARBOTHERMIC

ZIRCONIUM A.

OF

AND

ZIRCONIUM

REDUCTION

BORON

DIBORIDE

OF MIXTURES

OXIDES

I. Karasev

UDC 621.762.242:669.018.4:661.65

Of all r e f r a c t o r y compounds, z i r c o n i u m boride, ZrB2, o f f e r s the g r e a t e s t p r o m i s e . The fact that it r e t a i n s a c o m p a r a t i v e l y high s t r e n g t h at t e m p e r a t u r e s in e x c e s s of 1000~ has enabled it to be employed a s a component of h e a t - r e s i s t i n g alloys, the s o - c a l l e d b o r o l i t e s [1-5]; b e c a u s e of its good r e s i s t a n c e to c h e m i c a l attack by m o l t e n m e t a l s [2], it is a l s o used a s a h i g h - t e m p e r a t u r e r e f r a c t o r y [6] and in heat exc h a n g e r s of n u c l e a r r e a c t o r s . The m o s t c o m m o n method of m a n u f a c t u r e of z i r c o n i u m diboride, known as the b o r o n carbide p r o c e s s , is b a s e d on the r e a c t i o n [14] 2ZrO~ -~ B4C + 3C --- 2ZrB~ + 4CO. One of the m a i n d i s a d v a n t a g e s of this p r o c e s s is that the boron-containing m a t e r i a l used in it, b o r o n carbide, is r a t h e r expensive and in short supply. In a n o t h e r potentially m o r e useful method, z i r c o n i u m diboride is produced by the reduction of a m i x t u r e of Z r O 2 and ]3203 with c a r b o n a c c o r d i n g to the reaction ZrO~ + B203 + 5C = ZrB~ + 5C0. However, the a v a i l a b l e data show that, b e c a u s e of its volatility, a substantial e x c e s s of b o r i c anhydride is r e q u i r e d in this reaction. The method was f i r s t d e s c r i b e d by McKenna [8], who heated a m i x t u r e of ZrO2, B203, and c a r b o n in the m o l a r p r o p o r t i o n of 1 : 3 : 5 (i.e., with a 200% e x c e s s of b o r i c anhydride) in a graphite crucible at a t e m p e r a t u r e above 2000~ A detailed account of this method was l a t e r given by Blumenthal [9], who p e r f o r m e d the reaction, using a T a m m a n n furnace, in a s t r e a m of hydrogen. B l u m e n t h a l ' s study of the e x c e s s of b o r i c anhydride r e q u i r e d , r e v e a l e d that to l o w e r the c a r b o n content of the r e s u l t a n t boride to l e s s than 1%, a f o u r fold e x c e s s of B203 m u s t be employed [9]. S i m i l a r conclusions w e r e a r r i v e d at in investigations into the p r e p a r a t i o n of the b o r i d e s of r a r e r e f r a c t o r y m e t a l s [10-12]. According to S a m s o n o v [7], however it can be a s s u m e d that at c e r t a i n t e m p e r a t u r e s , it should be p o s s i b l e to find conditions u n d e r which the r a t e of reduction of z i r c o n i u m and boron oxides, with the f o r m a t i o n of z i r c o n i u m boride, e x c e e d s the r a t e of e v a p oration of b o r i c anhydride, so that the e v a p o r a t i o n is s u p p r e s s e d . In this connection, the p r e s e n t work was u n d e r t a k e n with the a i m of d e t e r m i n i n g the effects of t e m p e r a t u r e , holding time, and a m b i e n t a t m o s p h e r e and c h a r g e c o m p o s i t i o n s upon the p r o c e s s of p r e p a r a t i o n of z i r c o n i u m boride, ZrB2, by the reduction of z i r c o n i u m and b o r o n oxides with c a r b o n in T a m m a n n f u r n a c e s . As s t a r t i n g m a t e r i a l s , z i r c o n i u m dioxide (to MPTU 4355-53 specification) of 97.1% Z r O 2 content, b o r i c anhydride (to GO~T 10068-62 standard), and PM-50 g r a d e carbon black (to GOST 7885-68 standard) w e r e used. The z i r c o n i u m dioxide was s p r e a d in nickel t r a y s to f o r m beds 30-40 m m deep and then calcined f o r 2 h at 800~ to rid it of m o i s t u r e and organic contaminants. The z i r c o n i u m content of the calcined z i r c o n i u m dioxide w a s 73.3%. The c a r b o n black was calcined in s t e e l t r a y s for 2 h at 400~ a f t e r which its a s h content w a s found to be not m o r e than 0.3%. The b o r i c anhydride w a s ball m i l l e d for 15 h. The c h a r g e m a t e r i a l s w e r e m i x e d in a b a r r e l , o n e - t h i r d full, f o r 8 h at a c h a r g e - t o - b a l l weight r a t i o of 1 : 3, and the r e s u l t a n t m i x t u r e w a s rubbed through a sieve with a No. 045 b r a s s s c r e e n to b r e a k up any Kiev E n g i n e e r i n g Institute. T r a n s l a t e d f r o m P o r o s h k o v a y a Metallurgiya, No. 11 (131), pp. 80-84, N o v e m b e r , 1973. Original a r t i c l e submitted June 9, 1972. 9 1974 Consultants Bareau, a division o f Plenum Publishing Corporation, 227 ~'est 17th Street~ New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

926

T A B LE 1. V a r i a t i o n o f C h e r a i c a l C o m p o s i t i o n ride with Excess of Boric Anhydride in Charge,

of Z i r c o n i u m Diboafter Holding for

30 rnin at 2000~ Charge composition Zr0B:B,O~:C

B203

lexcess, [ Wt. %

1 --

1 : 1:5 1 : 1,1 : 5 1 : 1,2 : 5 1 : 1,3 : 5 1 : 1,4 : 5 1 : 1,5 : 5 1 : 1,8 : 5 1:2 :5 I :3 :5 l :4 :5 i : 5 :5

Chem. comp., wt. %

,IDeg" of Icompletehess of reaction, %

10 20 30 40 50 80 100 200 300 400

92,00 94,00 97,90 98,90 99,50 97,80 t00,10 100,06 100,12 100,12 100,17

Zr

B

81,50 80,00 80,30 79,60 78,10 82,00 81,50 81,20 81,00 80,50 80,00

16,10 17,60 18,30 18,00 17,20 16,90 14,40 16,00 17,00 18,70 I8,20

total C

Sum Z r+B+C

s Zr+B

1,70 0,55 0,74 0,70 0,70 0,80 0,80 2,80 1,50 0,75 0,70

99,30 98,15 99,34 98,30 9'6,00 99,70 96,70 100,00 99,50 99,95 98,90

16,50 18,00 18,60 18A0 18,00 17,10 15,00 17,00 17,30 18,90 18,50

T A B L E 2. C h e m i c a l Composition of Zirconiura Diboride at Various Holding Periods in Hydrogen at 2000~ Charge B203 Holding coml~osltion excess, time, ZrO2:B,Oa:C Wt., ~ min

1,5 : 5 1 : 1,1 : 5 l : 1,1 : 5 1 : 1,2 : 5 l : 1,2 : 5 1 : 1,2 : 5 1 : 1,3 : 5 1 : 1,3 : 5 I : 1,3 : 5 1 :

10 l0 10 20 20 20 30 30 30

60 120 240 60 120 240 60 120 240

co~%--I

Chem.

80,70 80,70 80,70 80,70 80,50 80,80 78,90 80,80 80,70

16,30 17,10 17,20 18,20 18,20 18,20 17,70 17,90 17,90

T A B L E 3. C h e r a i c a l C o m p o s i t i o n s Produced in Various Atmospheres

Produced

Sum Zr+B+C

1,20 0,60 1,10 0,77 0,23 0,74 1,02 0,23 0,92

97,00 97,80 97,90 99,67 98,93 99,74 96,60 98,70 98,60

B Zr+B

1'6,80 17,48 |7,56 18,40 18,43 18,38 18,32 18,30 18,50

of Zirconium Diboride Specimens at 2000~ from Charges with 20%

B20 3 Excess

Chem. comp., wt. % Atmosphere

Hydrogen Converted gas Without special atmosphere (carbon monoxide)

Zr

B

78,80 78,80

18,30 18,20

78,80

18,00

Sum

B

Zr4-B4-C

Zr+B

0,74 0,80

97,84 98,80

18,90 18,70

1,00

97,70

18A0

total C

pellets and obtain a homogeneous mass. The charge was then rammed into graphite containers, which were heated in a Tararaann furnace provided with a hydrogen atmosphere. Experiments were performed at temperatures ranging from I000 to 2000~ and a holding period of 30 rain. A period of 5-7 rain was required for the container temperature to reach the required level and become equalized along the whole length of the container. The results of the experiments, which were evaluated by cheraically analyzing the reaction products obtained to determine their Zr, B, and total C contents , are presented in Tables 1-3 and Figs. 1 and 2. The data in Table 1 and Fig. 1 show that Z rB 2 of technical purity can be obtained either at a B203 excess o f 1 0 - 3 0 % o v e r t h e s t o i c h i o r a e t r i c amount or, as has been demonstrated in the earlier investigations, at an excess

of 300-400%.

In both cases,

the resultant

zirconium

boride

contains

less

t h a n !~0 o f c a r b o n .

927

Zr+8§

1oo 90 82

f,! ~t,O

8/ 80

~5

gg,.

79 78

"c, ~a,e

19

2o,7

18 17

~o,4

16

e'~

~3

o

9,3 0,2

o

Et

on,!

~o

1ooo

Excess of B~Os, %

Fig. i Fig. i. Variation of chemical composition excess of B203 in charge.

i~00 i~00'15'00

17oo 2oo0

Temp., ~ Fig. 2

of zirconium diboride with

Fig. 2. Variation of degree of completeness of reaction with temperature in preparation of zirconium diboride by carbothermic method. Excess of B203 in charge: i) stoichiometric composition; 2) i0; 3) 20; 4) 30; 5) 40; 6) 50; 7) 80; 8) i00; 9) 200; 10)300; ii) 400%. Figure 2 depicts the effects of temperature upon the degree of completeness of the reaction, i.e., the ratio of the observed weight loss suffered by the charge to the weight loss corresponding to the reaction proceeding to completion. The external appearance and chemical composition of reaction products and the data in Fig. 2 show that at I000-1200~ the boride formation reaction does not occur at all. The reaction products obtained with a stoiehiometric charge and with charges containing a 10-30% excess of B203 are weakly sintered powder masses. Raising the temperature above this level initiates the boride formation reaction, and at 2000~ the resultant zirconium boride is of technical purity, with less than i% of carbon (except when a charge of stoichiometric composition is used). Clearly, at this temperature the B203 evaporation process is suppressed by the main boride formation reaction. The total amount of C in ZrB 2 produced at a 40-50% excess of B203 in the charge is also less than l~o. In this case however, the boric anhydride evaporates more intensely, and consequently zirconium boride obtained from a charge with the stoiehiometric amount of this compound contains less boron. At this excess of B203 in the charge, the rates of reduction of the zirconium and boron oxides are approximately equal. With an 80-400% excess of boric anhydride in the charge, a glass formation process takes place at temperatures of 1200-1800~ This phenomenon has already been observed in various metallic oxides mixed with substantial amounts of boric anhydride [13]. At 1800-2000~ the vitreous mass boils, clear evidence of which is provided by the appearance of the reaction products, the process being accompanied by intense evaporation and volatilization of the boric anhydride. Curves 7-11 in Fig. 2 indicate a sharp change in the degree of completeness of the reaction compared with curves 1-6. This leads to the conclusion that, at an 80-400~0 excess of boric anhydride, the rate of volatilization of the B203 grows with increase in the amount of this compound in the charge. Zirconium boride produced by the stoichiometric reaction contains more than I% of carbon because of the volatilization of the boric anhydride. The introduction into the charge of a 10-30% excess of B203 compensates for the loss of boric anhydride through evaporation from a charge of stoichiometric composition. Thus, at a 10-30% excess of B203 in the charge, the rate of reduction of the zirconium and boron oxides evidently exCeeds the rate of evaporation oftheB203. Utilizing this excess of boric anhydride in the charge, it is possible to obtain a zirconium diboride with a composition close to stoichiometric and a total C content of less than i%. As the excess of B203 in the charge is increased from 80 to 200~0, the rate of evaporation of the boric anhydride grows, making it impossible to produce a zirconium diboride with a sufficient amount of boron and 928

l e s s than 1% of total C. A f u r t h e r i n c r e a s e in the e x c e s s of b o r i c anhydride in the c h a r g e r e d u c e s the r e l ative amount of z i r c o n i u m dioxide p a r t i c i p a t i n g in the reaction; this enables z i r c o n i u m diboride to be p r o duced with a total C content of l e s s than 1% using a 300-400% e x c e s s of B203. Table 2 p r e s e n t s the r e s u l t s of e x p e r i m e n t s on the p r e p a r a t i o n of Z r B 2 at v a r i o u s t r e a t m e n t t i m e s and c h a r g e compositions. As can be seen, the b e s t c h e m i c a l c o m p o s i t i o n is exhibited by Z r B 2 produced by 2 hours reduction. It is, of c o u r s e , understandable that z i r c o n i u m and b o r o n oxides will be m o r e fully reduced in 120 rain than in 30 or 60 rain and a l s o that the r e s u l t a n t boride phase will be m o r e effectively homogenized throughout the c h a r g e . F o u r hours holding i n c r e a s e s the total C content of the boride, p r o b ably a s a r e s u l t of i m p r e g n a t i o n of the boride with c a r b o n f r o m the f u r n a c e a t m o s p h e r e . A study w a s m a d e a l s o of the influence e x e r t e d by the c o m p o s i t i o n of the g a s e o u s a t m o s p h e r e on the r e a c t i o n p r o d u c t s in the p r e p a r a t i o n of z i r c o n i u m diboride. A n a l y s e s of r e a c t i o n p r o d u c t s obtained in exp e r i m e n t s conducted in hydrogen and c o n v e r t e d g a s a t m o s p h e r e s and also without a special a t m o s p h e r e yielded r e s u l t s which a r e s u m m a r i z e d in Table 3. They show that replacing hydrogen with c o n v e r t e d gas in the m a n u f a c t u r e of z i r c o n i u m diboride has no a d v e r s e effects upon the reduction p r o c e s s . CONCLUSIONS A study was made of the p r e p a r a t i o n of technical z i r c o n i u m diboride by the reduction of m i x t u r e s of z i r c o n i u m and b o r o n oxides with c a r b o n u n d e r industrial conditions. It is shown that the o p t i m u m conditions f o r the p r e p a r a t i o n of z i r c o n i u m diboride by the c a r b o t h e r m i c method a r e established when a c h a r g e having the c o m p o s i t i o n ZRO2+1.2 B203+5 C (i.e., with a 20% e x c e s s of b o r i c anhydride o v e r the s t o i c h i o m e t r i c composition) is reduced at 2000~ in a hydrogen or c o n v e r t e d gas a t m o s p h e r e . The r e s u l t a n t Z r B 2 contains 18-19% B (compared with the t h e o r e t i c a l b o r o n content of 19.25%) and not m o r e than 0.8% C. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. !3. 14.

CITED

J. Campbell, M a t e r . Methods, 31, 59 (1950). I r o n Age, 173, 138 (1954). J. E v e r h a r t , M a t e r . Methods, 40, 90 (1954). F. G l a s e r , M e t a l P r o g r . , 67, 77 (1955). A. B l u m and W. Ivanick, P o w d e r Met. Bull., 7, 75 (1956). G . V . Samsonov, Ogneupory, No. 3, 122 (1956). G . V . Samsonov, Ukr. Khim. Zh., 24, No. 6, 799. P. McKenna, Ind. Eng. Chem., 28, 767 (1936). H. Blumenthal, P o w d e r Met. Bull., _7, 79 (1956). J . T . Norton, T r a n s . Met. Soc. A.I.M.E., 185, 749 (1949). G . A . Kudintseva, B. M. T s a r e v , and V. A. l~pel~baum, in: Boron, T r a n s a c t i o n s of a Conference on the C h e m i s t r y of Boron and Its Compounds [in Russian], GKhI, Moscow (1958), p. 106. R. Kieffer, F. Benesovsky, and E. Honak, Z. Anorg. Chem., 2.68, 191 (1952). W. G u e r t l e r , U e b e r W a s s e r f r e i e B o r a t e und u e b e r Entlassung, Leipzig (1904). G. u Samsonov, L. Ya. M a r k o v s k i i , et al., B o r o n and Its Compounds and Alloys [in Russian], Izd-vo Akad. Nauk UkrSSR, Kiev (1960), p. 377.

929