On the borothermic preparation of titanium, zirconium and hafnium diborides

~0~~~~2~1 ofthe Less-~o#~mff~ ~~~etals Elsevier Sequoia S.h., Lausanne - Printed in the Netherlands

ON THE BOROTHERMIC HAFNIUM DIBORIDES

PREPARATION

23

OF TITANII’M,

ZIRCONKM

AND

SUMMARY

The formation of titanium, zirconium and hafnium diborides by reaction between metal dioxides and elemental boron in vacuum in the temperature range 1ooo~--175o~C has been investigated. It has been established that titanium diboride with a composition close to that of stoichiometric TiBz is obtained after a heattreatment of the reactants at 1700°C for I h; HfB2 is obtained after heat treatment at 1750°C for z h. Experimental results show that higher tenlperatures are necessary for the preparation of stoichiometric ZrBz by the borothermic method. The structure of HfB2, and its stability at boiling point and at room temperature in the presence of acids, mixtures of acids, mixtures of acids with oxidizing agents, and alkaline solutions has been studied.

IIiTRODUCTIOS

.%mong the borides of the transition metals, the diborides of group WA of the Periodic Table are of special interest. They have high melting points and excellent physico-nlechanical properties’. Previous investigations~-4 have shown TiBz and ZrBz to be very stable with regard to various chemical reagents. On account of these properties the compounds show promise as materials for modern technical uses6*G. TiBz and %rBe have been studied in detail but there is little data in the literature concerning HfB2. The preparation of titanium and zirconium diborides has been described in a number of publications; papers on the preparation of HfBz are fewer in number however. The methods generally used can be divided into the following groups: (a) synthesis from the elements 7- 12; (b) electrolysis of molten oxides and saltsl3-15; (c) reduction with carbon of a mixture of metal dioxide (or the corresponding hydroxide) and Bz03 (or boric acid)l”-20; (d) solid-phase reactions which involve B&“*“1-“7; (e) synthesis from the metal and boron halides by a vapour deposition process according to the following reaction: MeX4 + 2B& -+5Hz = MeBz + IO HXz8-“0. Starting the introduction

from pure initial substances and with careful preparation (avoiding of impurities), pure products may be obtained by methods (a) and ,J. Less-Comman

Metuts, 14 (1908)

P. PESHEV,

24

G. BLIZNAKOV

(e). The high temperatures necessary for synthesis from the elements however, make this method inconvenient, whifst synthesis from the gas phase is very expensive. Products prepared by electrolysis, (b), always contain large amounts of impurities, whilst methods (c) and (d) lead to products containing smaller or larger amounts of carbon. The purpose of the present work was to examine the possibility of the preparation of pure titanium, zirconium and hafnium diborides using, as in previous investigation9-33, the borothermic reduction of oxides in vacuum: MeOz + 4B = MeBz + zB0

(1)

This reaction has already been used by PADERNO, SEREBRYAKOVA AND SAMSONOV~~ for the synthesis of HfBz, and by SEREBRYAKOVA AND SAMSONOV~~ for the preparation of TiBz and ZrBz. When this reaction takes place in an apparatus employing a graphite heater and a graphite crucible however, products containing carbon are obtained. Moreover, carbon probably causes, together with boron, the reduction of the oxide, which can be assumed from the lower preparation temperatures (by comparison with those found in the present investigation) established by the above authors. This situation will be discussed below. A preliminary estimate of the possibilities and temperature range of preparation of titanium, zirconium and hafnium diborides may be made thermodynamically. For this purpose, the dependence of the thermodynamic potential upon the temperatures of the borothermic reactions in the synthesis of TiB2, ZrB2 and HfB2 has been determined by the method used previously33. Since no definite data can be found in the literature on the enthalpy of formation of HfBz, its value has been calculated by the comparative method of I(APUSTIn’SK1135 using the values of the enthalpy of formation of TiB2 and ZrBz. The enthalpy of formation of HfBz thus determined is: AH’wJ = - 85.6 kcal/mol. Figure I shows the dependencies of the thermodynamic potential upon tem-

80

1500

2500

2000 TemperaturePi

Fig.

I. Change

borothermic

of thermodynamic

preparation

potential

with temperature

Fig. 2. Change of titanium content with increase in temperature aration of TiB2.

J. Less-Commn

Metals,

for the reactions

involved

in the

of TiB2, ZrBz and HfB2.

14

(1~68)

in specimens used for the prep-

BOROTHERMIC

PREPARATIOS

OF

Ti,

A?JI) Hf

13.0 rnzig; ZrOz, 6.2 m2/g; Hf02, 8.9 ms,:g; B, 53.2 m”/g. Stoichiometric mixtures of the oxides and boron were prepared according to eqn. (I), pressed into pellets and then heat-treated in a vacuum furnace using a molybdenum heater and a maximum working temperature of 1750°C. The working pressure was 10-4 mm Hg. The temperature was measured by an optical pyrometer. The pellets were contained in the reaction zone in fused alumina crucibles.

In the first series of experiments, the samples were heated at temperatures ranging between 1000~ and 1750°C for I h. X-ray phase analysis of the samples obtained was then carried out and the boron content of the resulting products and the amounts of titanium in TiB2 and unreacted TiOz determined by chemical analysis. In order to determine the boron content, the specimens were treated with 100/o Hz02 at boiling point after the addition of a few drops HNOa; the titanium of TiBz formed TiOz which was removed by filtration together with the initial unreacted TiOz. The boron was determined in the filtrate by titration with NaOH in the presence of mannitol and a mixed methyl red-pl~enolphtllalein indicator36s37. It was difficult to determine the titanium content of TiB2 on account of the great stability of Ti& towards chemical reagents. The usual method for the dissoluresults. tion of TiB2, in a mixture of H&04 (I : I) and I-TN03 37, led to unsatisfactory Jn the present investigation, perchloric acid was found to be the only reagent suitable for the complete dissolution of TiBt”; this reagent also has the advantage of not forming stable complexes with titanium. The solution obtained after treatment with HCIO4 was filtered in order to remove the unreacted TiOs; titanium was then precipitated with ammonia and determined as TiOe. Chemical analyses of the first series of products are given in Table I. The results show that the titanium content (in the form of TiB2) increases with increase in temperature. Up to 15oo’C this increase is comparatively slow, at 1600°C however, more than So:/, of the titanium in the resulting product is already combined in the form of TiB2, and at 1700°C the samples obtained have a composition chemical analysis indicates the close to the stoichiometric. At this temperature, absence of titanium in the form of TiOz. The change in the titanium content is shown in Fig. z. X-ray phase analysis confirms the presence of a mixture of TiOz and Ti% in the sample obtained at IOOO’ and ~roo”C. TiOa is present in the form of anatase.

26

P.

PESHEV, G. BLIZNAKOV

TABLE I CHEMICAL

COMPOSITION

No.

TemPerature (“Cl

1000

2

1100

3 4 5 6 ; 9

OBTAINED

Chemical

-ThBZ

I

OFSAMPLES

68.88* 21.68

IN THE

composition Ti Tie?

28.15

1200 1300 1400 1500 1600 1700

24.53 32.23 35.54 38.20

26.95 21.70

39.98 52.29 68.58

18.35 10.63

175”

70.51

* Theoretical

The X-ray anatase as Within the but lines of

19.77 18.85

PREPARATION

OF

TiBz

(%) B

(T$

31.12*

100.00*

31.38 30.53 31.48 31.48 30.46 29.34 29.99 30.15 30.04

+

T&o2

+

B)

81.21 82.01 84.51 86.79

87.51 87.67 92.20 98.73 100.55

composition

pattern of the product obtained at 1200°C displays lines of TiBz and well as those of the other polymorphic modification of TiOz, rutile. temperature range 13oo~-15oo~C the lines of anatase are already absent rutile are still observed. Lines of TiB2 alone are discernible above 17oo’C.

Preparation of zirconium diboride The heat treatment of the stoichiometric mixtures used for the preparation of ZrB2 had the same duration and was carried out at the same temperatures as in the preparation of TiB2. The selective solubility of the samples used for the preparation of ZrBz allows determination of both the whole boron content and also the free boron. The free boron was determined alkalimetrically after treatment of the specimens with 10% Hz02 and HN03 at boiling point and subsequent filtration (i.e., in the same way as the boron content in TiB2). To determine the total boron content in ZrB 239, the samples were dissolved by heating in a mixture of equal volumes of HzS04 (I : 2) and HzOz; the solution was then neutralized with NaOH and a hydroxide of zirconium precipitated; the precipitate was filtered off, H&04 was added to the filtrate and the boron was determined by titration with NaOH and mannitol. Zirconium was determined by dissolving the samples, with heating, in a mixture of H&04 (I : 4) and HN0$7. After removal of the insoluble residue, zirconium was precipitated with cupferron. The precipitate of zirconium cupferronate was ignited and the zirconium weighed as ZrO2. The results of chemical analysis of the samples obtained at the various temperatures are given in Table II. They show that the amount of combined boron increases very slowly up to 13oo’C; at 14oo’C a sharp increase in combined boron content occurs then again increases more slowly. At the maximum working temperature (1750°C) the content of combined boron in the sample is still smaller than the theoretical amount of boron in ZrBz. The changes in the content of zirconium, and of free- and combined boron are given in Fig. 3. X-ray phase analysis indicated lines of ZrBa at 1200°C. At 1000’ and IIOO’C the X-ray patterns showed lines of monoclinic ZrO2 alone, which is stable within J. Less-Common

Metals,

14 (1968)

PREPARATION

BOROTHERMIC

CHEMICAL

COMPOSITION

OF

OF SAMPLES

Ti, Zr AND Hf IHBORIIES

OBTAINED

IN

THE

PREPARATION

OF .&Y&

__~_.

_vo

Temperature /ori

,

Chemical

_..

-I

.

composition

Bfrw _. ~~~.

2r

Bcom(iin~a

3

I200

4

IPJ

EkJ.s3* 58.93 58.67 59.79 Q.51

5 6 :

1400 1500 I 600

71.18 73.96 77.10

9.05 7.00 4.82

1700 7750

78.80 XI,45

2.40

r4.13

9

,.I0

IS.59

I

IO00

2

II00

* Theoretical

21.92 21.51 19.83 16.80

(O/u)

___~__.__

0.2.9

o.bb 1.x1 2.10 9.58 9.93 IX.61

composition.

a -6op; -50 5 L 8

1000

1200

1400

Temperature

1600

1000

1200

PC)

Fig. 3. Change in the content of zirconium, and free- and combined perature, in samples used for the preparation of ZrB?. Fig. 4. Change in the content of Hf, and free- and combined in samples used for the preparation of HfBz.

1400

1000

TempemturePC)

boron with incrcasc in tem-

boron, w+th increase in temperature,

the whole temperature range investigated. Lines of monoclinic ZrOz were observed up to 1600°C, and at higher temperatures the only phase identified was ZrB2. At the maximum working temperature (1750°C) a product containing less combined boron than the theoretical, in addition to a certain amount of unreacted free boron, was obtained. It is evident that the rate of the solid phase reaction used for the preparation of ZrBz is very slow and in consequence part of the boron in the starting mixture is evolved before it is has time to react. Mixtures containing boron in excess of the stoichiometric amount were heated at 1750°C for 2 h. With the gradual increase of excess boron up to IO%, the percentage of combined boron in the samples increased a little, the amount of free boron remaining approximately constant. Chemical analysis confirmed the following composition of the product obtained from a starting mixture containing 10% of excess boron : Zr, 81.46% ; Bcombined,x6.55:/0 ; Brree, 1.10%; E&M, 17.65~/& J. Less-Common Metals, *Q (rg68)

P. PESHEV, G. BLIZWAKOV

2s

Greater amounts of excess boron in the initial mixture led to an undesirable increase in the amount of free boron in the reaction product. On the basis of the results given above, it may be concluded that higher working temperatures, unattainable by means of the apparatus used in the present work, are necessary for the preparation of stoichiometric ZrB2 by the borothermic method. Prefuration, structureand jmperties of HfB2 The series of stoichiometric mixtures of HfOe with boron was heat-treated in vacuum, in the same manner as above, at temperatures ranging from 1000~ to 1750°C. It was necessary to know the chemical properties of HfBa in order to carry out analysis of the samples obtained. There are little data in the literature concerning these properties and it was necessary to study them in detail. The results of this investigation which will be discussed below, showed that analogous methods and reagents to those employed for the analysis of the samples used in the preparation of ZrBs may also be applied to the determination of Hf, and total and free boron. Table III shows the results of chemical analysis of samples used in the preparation of HfB2, while Fig. 4 illustrates the change in the contents of Hf, and free and combined boron. TABLE

III

CHEMICAL.COMPOSITIONOF SAMPLES

I-_No.

Telnpevature

(“C)

OBTAINED

IN THE

Bfrce

Hf

B~D~bZ?l~d Bw,t

sg.rg* 1000

68.17

14.12

0.27

2

1100

68.87

3

1200

4

1300 1400

7447

13.77 13.22

0.53 0.74 0.76 6.09

1500 IG

1600 1700

9**

‘750

-

OF

HfBz

Ckenzical composition (“/b)

I

2

PREP?IRATION

12.77 6.26 I.91 0.89 0.25

75-49 78.19 83.2’ 84.50 87.93 88.37

* Theoretical composition. ** Duration of heat treatment:

2

-- (Hf

+ Btotaz)

1o.s1*

100.00*

‘4.39 14-30 73.96

83.17

9.03 9.65 10.58

73.53 ‘3.23 IO.94 ‘O?i4 10.83

10.83

10.83

82.56 aa.43 89.02

91.42 94.15 95.04 98.76 99.20

hours.

X-ray phase-analysis confirmed that the only phase which can be identified up to 1200DC is monoclinic IIfOz; at 13ooYZ lines other than those of HfOs are apparent. There are no data in the literature concerning lattice-spacings of HfBe; it is known only that HfBz has a hexagonal structure. The X-ray patterns of products prepared at higher temperatures show an increasing intensity of lines corresponding most probably to the HfBg-phase. On the basis of the X-ray patterns of samples treated at r600”-1750°C it may be concluded that a product which is isomorphous to TiBs and ZrBs is obtained; this without doubt is HfBa. The lattice-spacings and the intensity of the lines of HfB2 obtained at 1750°C are presented in Table IV. As mentioned above, there is little known concerning the behavior of HfBz towards various chemical reagents: only the extent of dissolution of HfBz obtained

BOROTHERMIC

PREPARATION

OF

Ti, Zr AND Hf

29

DIBORIIIES

by the borocarbide method (treatment at boiling point for I h in concentrated HN03 and HCI and diluted I:I solutions of the same acids) has been investigated previously37. It has been established that HfBz is only partially dissolved by these reagents. The extent of the dissolution of HfB2 in various acids, mixtures of acids, TXIXLE IV X-RAY DATA

ON

HfBz

CuKx radiation/nickel x0. I

2 3 4 5 6 ; 9 IO

filtered at 35 kV and 12 mA. Exposure

I

2L (mm)

d (A)

6 9 IO

25.9 33.3 42.7 53.’ 59.4 63.9 65.7 69.4 75.5 83.3

3.51 2.73 2.14

3 4 5 4 3 5 5

1.736 I.564 1.463 I.427 1.359 1.262 1.162

time 3 h.

x0.

I

ZL (mm)

d f&

II

7

93.0 97.6 103.3 112.1 116.9 121.9

I ,063 1.025 0.9828 0.9287 0.9038 0.8808 0.8746 0.8665 0.8245 0.8021

12 I3 ‘4 ‘5 16 17 18 19 20

04 3 3 7 I 0.5 5 7

“3.4 125.4 138.0 ‘47.3

mixtures of acids with oxidizing agents, and alkaline solutions has been investigated in detail in the present work. The same reagents and conditions of treatment as in previous investigations of the chemical properties of TiB2 and ZrB&3 were used, with a view to comparing the results. The product utilized for the investigations had a specific surface of about 2 m2/g. To obtain a semi-quantitative estimate of the effect of acids and acid mixtures, samples of about 0.1 to 0.2 g HfB2 were treated with the particular reagents at room temperature, or at boiling point, in a flask connected with a reflux condenser. The insoluble residues were filtered off through a Gooch filter, washed with the acid used as solvent, then with distilled water, subsequently dried, and weighed. TABLE

V

INSOLUBLE RESIDUE, (?/,), AFTER TREATMENT OF HfBz WITH

ACIDS* __~~____

No.

Insoluble

Reagent

residue

(“b)

After treatment at boiling point for2 h

IO

* Weight

H&04 (d = 1.84) HzS04 (I: 4) HN03 (d = 1.40) HN03 (I : I) HCl (d = 1.19) HCl (I : I) HClOd (d = 1.65) HClOa (I : 3) H&204 (a saturated solution at 2o’C) &PO4 (I: 4) of the samples:

0.1 g;

1.87 3.84 2.98 3.24 6.37 5.65 2.54 36.85 5.05

I,oo.oo

Aftev tveatnzent at 20°C fou24 h 91.58 96.07 47.87 54.35 97.70 96.60 IOO.00 9j.12

93.94 97.5’

volume of the acid: 25 ml J. Less-Common

Metals,

14 (1968)

P. PESHEV,

30

G. BLIZNAKOV

In the first series of experiments, the extent of dissolution of HfBz treated in acids at boiling point for 2 h and at 2oT for 24 h was examined. The results obtained are given in Table V. They show that at the boiling point HfBz is affected most strongly by concentrated H&O4 whereas, at room temperature, it is attacked most strongly (although to a smaller extent than by the former) by concentrated nitric acid. A comparison with data on the extent of dissolution of TiBz and ZrBz (under the same conditions) shows that HfBs is more strongly affected by acids than is TiB2. ZrBz displays a certain peculiarity in this connection; at room temperature it is in general attacked more strongly than HfB2, but at the boiling point the opposite holds, specimens of HfBz dissolving to a greater extent than those of ZrBz. The data on the extent of dissolution of HfBz in mixtures of acids and in mixtures of acids and oxidizing agents are given in Table VI. TABLE

VI

INSOLUBLE REsiDUE (%) OF ACIDS

No.

AND

AFTER TREATMENT

OF

HfBz

WITH MIXTURES

OF ACIDS

AND

MIXTURES

OXIDIZERS*

Ratio between the components

Reagent

HCl (d = 1.19) + HNOz (d = 1.40) HCI (I:I) + HNOa

Insoluble residue ( yO) after treatment at b.p. for 2 h

after treatment at 20°C for 24 h

3:1

?.31

5.95

3:1

2.93

28.81

x:1:*

2.57

3.30

3:2

2.94

96.32

35 ml + 15 ml

3.13

29.05

35 ml + 15 ml 1:4:2

3.00

91.24

5.43

93.43

0

99.21

0

33.17

(I : I)

4

8 9 IO

* Weight

H&204 (saturated solution at 2o°C + HzOz (30%) + HNOz (d = 1.40) H&z04 (saturated solution at 2o’C) + H&04 (d = 1.84) HCI (d = 1.19) + bromine water HC104 (d = 1.65) + HCl (d = 1.19) H&04 (d = 1.84) + H3P04 (d = 1.42) +

Hz0

HzS04 (d = I&SO4 HzS04 (d =

KzSzOs

H&04 HN03

1.84) + 1.84) +

(d = 1.84) + (d == 1.40)

of the samples:

5 ml + 5g 5 ml + 5g 35 ml + 15 ml

0.2 g; volume

of acid:

1.87

2.09

50 ml.

As is evident from the data in the above Table, HfBz dissolves completely only when treated at boiling point with mixtures of concentrated H&04 and K&04, or concentrated H&O4 and K&08. For the sake of comparison it is worth mentioning that titanium and zirconium diborides cannot be completely dissolved by the reagents given in Table VI. J. Less-Common Metals, 14 (1968)

ROROTHERMICPREPARATIONOF Ti, Zr .~ND Hf DIBORIIIES

31

The stability of HfBz towards aqueous NaOH has also been investigated. Samples of 0.1 g were treated with 25 ml of 307; NaOH at the boiling point for z h, and at room temperature for 24 h. The results of these experiments showed that HfBa is only partially soluble in NaOH, insoluble salts being formed. The dissolution proceeds to a greater extent when a treatment at boiling point, using 3004 NaOH, is carried out.

The experimental results confirm the possibility of the formation, by the borothermic method, of titanium and hafnium diborides with compositions close to the stoichiometric, at 1700’ and 175O”C, respectively. ZrBz may probably be obtained at higher temperatures. The data obtained are in agreement with the thermodynamic calculations which gave the order of temperatures necessary for the preparation of the three borides. The temperatures of preparation established in the present work are higher than those found by previous authorP,34 ranging between 1600’ and 1700°C. Taking into consideration the experimental conditions used by these workers it may be assumed that the following reaction takes place simultaneously with reaction (I) : MeOz+zB+2C=MeBz+zCO

(2)

Calculations of the change in the thermodynamic potential with temperature (with the preparation of diborides in the presence of boron and carbon) show that the reactions should really take place at lower temperatures (Fig. 5).

1000

1500

2000

2500

Temperatui~(‘K)

Fig. 5. Change of thermodynamic potential with temperature for the reaction fire02 -+ rB + IC = IvIeBz+ aC0.

ACKKOWLEDGEMENTS

The authors with to express out the X-ray analyses.

their thanks

to Mr. I. TSOLOVSKY for carrying

,I. Less-Common

Metals,

I.+ (1908)

P. PESHEV, G. BLIZNAKOV

32 REFERENCES I

G. V. SAMSONOV, Plenum Press Handbooks of High Tem+fature Materials, No. 2, Properties Index, Plenum Press, New York, 1964. K. D. MODYLEVSKAYA AND G. V. SANISONOV, Ukr. Kkim. Zk., 25 (1959) 55. L. YA. MARKOVSKII AND G. V. KAPUTOVSKAYA, Zk. Neorg. Kkim., 3 (1960) 569. R. MEYER AND H. PASTOR, Bull. Sot. Fr. Ceram., (66) (1965) 59. R. STEINITZ, in H. H. HAUSNER (ed.), Modern Materials, Vol. 2, Academic Press, New York, 1960, PP. IgI--224.

6 R. THOMPSON, Borides: Their Chemistry and Application, The Royal Institute of Chemistry, Lecture Series No. 5. London, 1965. A. MOISSAN. Compt. Rend., 70 (1895) 290; Ann. Ckim. Pkys., 7 (1896) 229. ; E. WEDEKIND, Ber., 46 (1913) 1198. 9 P. ERLICH, Z. Anorg. Allgem. Ckem., 259 (1949) I. 10 E. DECKER AND J. KASPER, Acta Cry&, 7 (1954) 77. II A. POLTY, H. MARGOLIN AND J. NIELSEN, Trans. Am. Sot. Metals, 46 (1954) 312. 12 V. A. EPEL’BAUM AND M. A. GUREVICH, Zh. Fis. Kkim., 32 (1958) 2275. 13 L. ANDRIEUX, Ama. Ckim. Pkys., 12 (1929) 42; J. Four. E&W., 57 (1948) 54. ‘4 F. GLASER, Powder Met. Bull., No. 6 (1951) 51. I.5 S. SIADEBAND AND P. SCHWARZKOPF, Powder Met. B&l., *No. 5 (rggo) 42. 16 P. MCKENNA, f%d. Eng. Ckem., 28 (1936) 767. 17 G. A. KUDINTSEVA, B. M. TSAREV AND V. A. EPEL’BAUM, Proc. Conj. a% Boron, its Chemistry and its Compounds, Moscow, Dec. rg55, Goskhimizdat, Moscow, 1958, p. ro6. 18 T. ATODA, S. YAXAGUCHI AND S. KITAHARA, Sci. Papers Inst. Pkys. Gkem. Res. (Tokyo), 53 (1959) 68. I9 U.S. Pat. 2973247 (1961). 20 H. BLUMENTHAL, Powder Met. Bull., No. 7 (1956) 79. 21 R. KIEFFER, F. BENESOVSKY AND E. HONAK, Z. Anorg. Allgem. Gkem., 268 (1952) IQI. 22 T. KUBO AND T. HANAZAWA, Kogyo Kagaku Zasski. 63 (1960) 1144. 23 V. F. FUNKE, S. I. YUDKOVSKII AND G. V. SAMSONOV, Zh. Prikl. Khim., 33 (1960) 831. 24 G. A. MEERSON AND G. V. SAMSONOV, Zk. Pvikl. Khim., 27 (1954) 1135. 25 C. BARUCH AND T. EVANS, J. Metals, 7 (1955) 909. 26 V. F. FUNKE AND S. I. JUDKOVSKII, Povoshkovaya Met., Akad. Nauk Ukr. SSR, 3 (4) (1963) 49. 27 Yu. B. PADERNO, T. J. SEREBRYAKOVA AND G. V. SAMSONOV, Tsvetn. Metal., (II) (1959) 48. 28 K. MOERS, Z. Anorg. Allgem. Ckem., 198 (1931) 243. 29 R. E. GANNON, R. C. FOLWEILER AND ‘I. VASILOS, J. Am. Ceram. SOL, 46 (1963) 496. 30 P, PESHEV, Mitt. Inst. Allgem. Anorg. Chem. {Sofia), 4 (1966) 53. 3’ G. BLIZNAKOV AND P. PESHEV, J. Less-Common Metals, 7 (1964) 441. 32 G. BLIZNAKOV, P. PESHEV AND I.. LEYAROVSKA, Cornet. Rend. Acad. Bzllg. Sci., 19 (1~66) 381. 33 P. PESHEV, G. BLIZXAKOV AND L. LEYAROVSKA, j. Less-cornrn~ Metals, r3 (1967) 241-247. 34 T. I. SEREBRYAKOVA AND G. V. SAMSONOV, Ukr. Khim. Zk., 29 (1963) 876. 35 A. F. KAPUSTINSKII, Izv. Akad. Nauk SSSR, Otd. Kkim. Nalck, (1948) 568, 581. Haadbuck der analyt~scke~ Chemie, Teil III, 36 E. WIEBERG, Analyse der Borverbindungen, Band III, 1942. 37 G. V. SAMSONOV (ed.), Analysis of Refractory Compounds, Metallurgizdat, Moscow, 1962. 38 L. N. KUGAI AND T. N. NAZARCHUK, Zk. Aleal. Khim., 16 (1961) 205. 39 V. G. SHCHERBAKOV, R. M. VEITSMAN AND ‘2. K. STEGENDO, Tr. Seminara $0 Zkarostoikim Materialam, Akad. Nauk Ukr. SSR, Inst. Metallokeram. i Spets. Splavov, Kiev, ~960, 1961 No. 6, P. 52.

J. Less-Common Metals, 14 (1968)