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On the preparation of niobium arsenides and antimonides
SHORTCOMMUNICATIONS
227
The authors are grateful to Mr. A. D. FRIDAY for taking the electron micrographs. This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio, under Contract No. AF33(657)-10363. Battelle Memorial
A. GILBERT W. R. WARKE B. A. WILCOX
Institute
505 King Avenue, Columbus, Ohio (U.S.A.)
1 G. T. HAHN, A. GILBERT AND R. I. JAFFEE, in Refractory Metals and Alloys, II, (Mctallorgical Society Conferences, Vol. 17), Interscience, New York, 1963, pp. 23-63. 2 W. C. COONS, The Metal Molybdenum, Am. Sot. Metals, Cleveland, Ohio, 1958. pp. 394-407. 3 M. SEMCHYSHEN,G. D. MCARDLE AND R. Q. BARR, Development of high strengths and higher recrystallization temperatures in molybdenum-base alloys, W.A.D.C. Tech. Rep. 58-551, hSTIA Document No. 209383, Feb. rg5g. .L A. A. GRIFFITH, Proc. First Intern. Congr. Appl. Mech., Delft, '924, p. jj. 6 H. NEUBER, Theory of Notch Stresses, Springer Verlag, Berlin, 1955; AEC-Tech. Rept.-4547, p. 76. 6 C. E. INGLIS, Trans. Inst. Avaval. Arch. (Londolz), 55 (1913) 219.
Received March Ipth, 1964 J. Less-Common
On the preparation
Metals, ZZZ--LL~
of niobium arsenides and antimonides
In a recent study of niobium arsenides and antimonides the existence of compounds with the compositionNbAs,NbAsz,NbsSb,NbsSb4,andNbSbzhasbeen established and their crystal structures and magnetic properties determinedI_4. The crystal structure of NbAs has been studied independently by BOLLER AND PARTH~~ and the present authors. The results for NbsSb confirmed the findings of MATTHIASet aZ.6, WOOD et al.7 and NEVITT**. While NbAsz and NbSbz were easy to prepare, the nature of the niobium metal was found to be of importance in the formation of the compounds NbAs, NbaSb and Nb&br. Samples were prepared from three different batches of niobium: fine-powdered spectrographically standardized niobium (Johnson, Matthey & Co., Ltd.), coarsegrained spectrographically standardized niobium powder (Johnson, Matthey & Co., Ltd.) and niobium sheet (Murex Ltd.); high-purity arsenic (American Smelting and Refining Co.) and high-purity antimony (Bradley Mining Co.) were used. For niobium contents exceeding N 50 at. y0 the use of the fine-powdered niobium led to contamination of the arsenides and antimonides by NbO and Nb&ia and, additionally, by SiO2 in the case of the antimonides. The formation of NbO andNb& can be explained by the tendency of niobium to react with silica. According to SCHAFER * Note added in proof: The paper by SAINI et al.11 on the niobium-arsenic system has come to hand since this manuscript was submitted for publication. Their paper independently confirmed the existence of NbAs and NbAsz. J. Less-Common
Metals, 7 (1964) 227-230
228
SHORTCOMMUNICATIONS
AND DOHMANN~this reaction is catalysed by the presence of hydrogen, traces of which are present in the fine-powdered niobium. Alumina crucibles placed inside the silica tubes, which have often been used with success in this Institute, did not lead to any improved results since hydrogen acts as a transport agent for 50s. The reaction with silica was avoided by the use of hydrogen-free coarse-grained niobium. In this case, however, the samples had not reached equilibrium: Guinier photographs of samples with a metal content higher than N 60 at. y0 Nb clearly showed the presence of unreacted niobium. A microscopic study of metallographic specimens prepared from some of the largest particles in these samples revealed a nucleus of unreacted niobium covered with an adherent niobium pnictide coating. This finding prompted the use of niobium sheet. The sheet was cut into specimens measuring 8 x 6 x I mm. Prior to use, the specimens were polished and rinsed in distilled water and alcohol. The samples were prepared by heating the weighed quantities of niobium and arsenic or antimony, respectively, in evacuated and sealed silica tubes which had been shaped to prevent contact between the niobium metal and arsenic or antimony except via the gaseous phase. The reaction was started by dropping the samples in a furnace at the desired temperature. After the heating the reaction was stopped by quenching the silica tubes in ice water. Metallographic specimens were prepared using araldite as mounting material. After grinding and polishing with levigated alumina or a fine-grained diamond paste, transverse sections of the specimens were examined with a Reichert universal camera microscope. The results were the same as those for the coarse-grained niobium powder, i.e. firmly adherent coatings which did not spa11from the nucleus during or after cooling of the specimens to room temperature. The relative thickness of these coatings depended either on temperature and heating time or on the atomic ratio Nb : X (X = As or Sb). For an atomic ratio Nb : X > I : 2 and a heating time sufficiently long to ensure complete conversion of all arsenic or antimony (i.e. when no visible amounts of the metalloids were seen) the thickness of the unreacted nucleus of niobium always bore a simple relation to the Nb : X ratio. The thickness of the unreacted nucleus corresponds to a reaction of the type: Nb + Xz =
NbXz
Since only one pnictide layer was plainly visible on the metallographic crosssections this was expected to be NbX2. Guinier photographs of the abraded pnictide layer confirmed this hypothesis. An extremely thin inner layer next to the metal nucleus of a different composition might nevertheless be present, as some of the metallographic specimens show. The thickness of the unreacted nucleus did not change on annealing, whereas recrystallization occurred in the niobium pnictide layer. The reason for the apparently non-equilibrium conditions in the niobium pnictides made from coarse-grained niobium is undoubtedly of a kinetic character. The situation is parallel to the preparation of platinum “mono”-chalcogenides, as described by KJEKSHUS~~. The same evidently applies to a great many transition metal chalcogen and pnigogen systems. Some kinetic data for the reactions between niobium and arsenic or antimony, respectively, have been collected. The same experimental technique was used, but with atomic ratios Nb:X
Metals,
7 (1964)
227-230
229
SHORT COMMUNICATIONS
measurements on each sample) of the niobium pnictide layer relative to the original thickness of the niobium sheets (in these experiments sheets of constant thickness were used) as a function of the reaction time. The results show that in the temperature range 7oo-925°C the reaction between niobium and antimony follows a linear rate law. This suggests that the antimonide scales formed under these conditions have very poor protective properties. The metallographic cross-sections which showed porous NbSbz scales with comparatively large holes are consistent with this view.
20
40
60
80
Time
(hours)
100
120
140
---w
Fig. I. The reactions between niobium and arsenic at 7zoY (0) and between niobium and antimony at 700°C (Cl), 800” (v) and 925’C (a). Relative thickness of niobium pnictide layer as a function of time.
In the same temperature interval the reaction between niobium and arsenic seems to be difficult to interpret in terms of a definite parabolic or linear rate law (see the 720°C run shown in Fig. I). The metallographic cross-sections showed that the NbAsz scales were more dense and more compact than the ~~)rrespondingNbSbz scales. The growth habits of the scales also differ in another way. The arsenide specimens showed a preferential tendency to react at the edges of the metal cores with a subsequent cleavage of scales at the edges. Micro-indentation hardness (measured with a Vickers pyramid at a single load of 42 g) recorded over transverse sections of the NbAsz scales showed a gradual increase from 1400kg/mm2 at the metal/gas interface to a constant value of 2400 kg/mm2 obtained at about 0.08 mm from the interface. Similar measurements carried out on the NbSbz scales were not reproducible in agreement with the porosity and holes in these scales. The micro-indentation hardness (MHs, = 770 kg/mm2, MHioll = 505 kg/mm” and MH2, = 340 kg/mm”) in the metal core did not depend upon the reagent (arsenic or antimony), the temperature or the duration of the reaction and was constant throughout the metal core. The pnigogen penetration into the metal core or the diffusion of metal outwards is thus rather small. As seen from Fig. I the reactions between niobium and arsenic or antimony leading to the formation of the NbX2 compounds are fairly fast. For most forms of niobium
SHORT COMMUNICATIONS
230
starting material the reactions will be completed and equilibrium obtained in some days. With regard to the preparation of NbAs, Nb&b and Nb&bd, which was one of the main objects of this study, our conclusions are: (i) These compounds cannot be prepared by a direct reaction from the elements using coarse-grained niobium. The resulting products are heterogeneous, with niobium and niobium di-pnictides as the main constituents. (ii) The compounds might be obtained from sufficiently powdered niobium. However, traces of hydrogen present in the niobium produce considerable amounts of impurities (NbO, Nb5Si3 and crystalline SiOz). (iii) NbAs, NbaSb and NbsSb4 are best prepared by thermal decomposition of NbAsz and NbSbz, both of which are easily made. According to FURUSETH AND KJEKsHus3,
NbAs
NbsSb4
Nb&b
and
is obtained by
after complete
degradation
of NbSba
degradation at 830°C
of NbAsz and
IOOO’C
at IIOO’C
and
respectively.
The authors wish to thank Professor H. HARALD~EN for his interest in this study and for placing
laboratory
Kjemisk Institutt A, University of Oslo, Blindem (Norway)
facilities
at their disposal.
S. FURUSETH A. K JEKSHUS
1 S. FURUSETH AND A. KJEKSHUS, Nature, in press. 2 S. FURUSETH AND A. KJEKSHUS, Acta Cryst., in press. 3 S. FURUSETH AND A. KJEKSHUS, Acta Chem. &and., in press. 4 S. FURUSETH AND A. KJEKSHUS, Acta Cryst., in press. 5 H. BOLLER AND E. PARTHB, Acta Cryst., 16 (1963) 1095. 6 B. T. MATTHIAS, E. A. WOOD, E. CORENZWIT AND V. B. BALA, J. Phys.Chem. Solids, I (1956) 188. 7 E. A. WOOD, V. B. COMPTON, B. T. MATTHIAS AND E. CORENZWIT, Acta Cryst., II (1958) 604. 8 M. V. NEVITT, Trans. AIME, 212 (1958) 350. 8 H. SCHAFER AND K.-D. DOHMANN, Z. Anorg. ALlgem. Chem., 2gg (1959) 197. 10 A. KJEKSHUS, Acta Chem. Scand., 15 (1961) 159. 11 G. S. SAINI, L. D. CALVERT AND J. B. TAYLOR, Can. J. Chem., 42 (1964) 630.
Received March Irth, 1964 J. Less-Common Metals, 7 (1964) 227-230
227
The authors are grateful to Mr. A. D. FRIDAY for taking the electron micrographs. This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio, under Contract No. AF33(657)-10363. Battelle Memorial
A. GILBERT W. R. WARKE B. A. WILCOX
Institute
505 King Avenue, Columbus, Ohio (U.S.A.)
1 G. T. HAHN, A. GILBERT AND R. I. JAFFEE, in Refractory Metals and Alloys, II, (Mctallorgical Society Conferences, Vol. 17), Interscience, New York, 1963, pp. 23-63. 2 W. C. COONS, The Metal Molybdenum, Am. Sot. Metals, Cleveland, Ohio, 1958. pp. 394-407. 3 M. SEMCHYSHEN,G. D. MCARDLE AND R. Q. BARR, Development of high strengths and higher recrystallization temperatures in molybdenum-base alloys, W.A.D.C. Tech. Rep. 58-551, hSTIA Document No. 209383, Feb. rg5g. .L A. A. GRIFFITH, Proc. First Intern. Congr. Appl. Mech., Delft, '924, p. jj. 6 H. NEUBER, Theory of Notch Stresses, Springer Verlag, Berlin, 1955; AEC-Tech. Rept.-4547, p. 76. 6 C. E. INGLIS, Trans. Inst. Avaval. Arch. (Londolz), 55 (1913) 219.
Received March Ipth, 1964 J. Less-Common
On the preparation
Metals, ZZZ--LL~
of niobium arsenides and antimonides
In a recent study of niobium arsenides and antimonides the existence of compounds with the compositionNbAs,NbAsz,NbsSb,NbsSb4,andNbSbzhasbeen established and their crystal structures and magnetic properties determinedI_4. The crystal structure of NbAs has been studied independently by BOLLER AND PARTH~~ and the present authors. The results for NbsSb confirmed the findings of MATTHIASet aZ.6, WOOD et al.7 and NEVITT**. While NbAsz and NbSbz were easy to prepare, the nature of the niobium metal was found to be of importance in the formation of the compounds NbAs, NbaSb and Nb&br. Samples were prepared from three different batches of niobium: fine-powdered spectrographically standardized niobium (Johnson, Matthey & Co., Ltd.), coarsegrained spectrographically standardized niobium powder (Johnson, Matthey & Co., Ltd.) and niobium sheet (Murex Ltd.); high-purity arsenic (American Smelting and Refining Co.) and high-purity antimony (Bradley Mining Co.) were used. For niobium contents exceeding N 50 at. y0 the use of the fine-powdered niobium led to contamination of the arsenides and antimonides by NbO and Nb&ia and, additionally, by SiO2 in the case of the antimonides. The formation of NbO andNb& can be explained by the tendency of niobium to react with silica. According to SCHAFER * Note added in proof: The paper by SAINI et al.11 on the niobium-arsenic system has come to hand since this manuscript was submitted for publication. Their paper independently confirmed the existence of NbAs and NbAsz. J. Less-Common
Metals, 7 (1964) 227-230
228
SHORTCOMMUNICATIONS
AND DOHMANN~this reaction is catalysed by the presence of hydrogen, traces of which are present in the fine-powdered niobium. Alumina crucibles placed inside the silica tubes, which have often been used with success in this Institute, did not lead to any improved results since hydrogen acts as a transport agent for 50s. The reaction with silica was avoided by the use of hydrogen-free coarse-grained niobium. In this case, however, the samples had not reached equilibrium: Guinier photographs of samples with a metal content higher than N 60 at. y0 Nb clearly showed the presence of unreacted niobium. A microscopic study of metallographic specimens prepared from some of the largest particles in these samples revealed a nucleus of unreacted niobium covered with an adherent niobium pnictide coating. This finding prompted the use of niobium sheet. The sheet was cut into specimens measuring 8 x 6 x I mm. Prior to use, the specimens were polished and rinsed in distilled water and alcohol. The samples were prepared by heating the weighed quantities of niobium and arsenic or antimony, respectively, in evacuated and sealed silica tubes which had been shaped to prevent contact between the niobium metal and arsenic or antimony except via the gaseous phase. The reaction was started by dropping the samples in a furnace at the desired temperature. After the heating the reaction was stopped by quenching the silica tubes in ice water. Metallographic specimens were prepared using araldite as mounting material. After grinding and polishing with levigated alumina or a fine-grained diamond paste, transverse sections of the specimens were examined with a Reichert universal camera microscope. The results were the same as those for the coarse-grained niobium powder, i.e. firmly adherent coatings which did not spa11from the nucleus during or after cooling of the specimens to room temperature. The relative thickness of these coatings depended either on temperature and heating time or on the atomic ratio Nb : X (X = As or Sb). For an atomic ratio Nb : X > I : 2 and a heating time sufficiently long to ensure complete conversion of all arsenic or antimony (i.e. when no visible amounts of the metalloids were seen) the thickness of the unreacted nucleus of niobium always bore a simple relation to the Nb : X ratio. The thickness of the unreacted nucleus corresponds to a reaction of the type: Nb + Xz =
NbXz
Since only one pnictide layer was plainly visible on the metallographic crosssections this was expected to be NbX2. Guinier photographs of the abraded pnictide layer confirmed this hypothesis. An extremely thin inner layer next to the metal nucleus of a different composition might nevertheless be present, as some of the metallographic specimens show. The thickness of the unreacted nucleus did not change on annealing, whereas recrystallization occurred in the niobium pnictide layer. The reason for the apparently non-equilibrium conditions in the niobium pnictides made from coarse-grained niobium is undoubtedly of a kinetic character. The situation is parallel to the preparation of platinum “mono”-chalcogenides, as described by KJEKSHUS~~. The same evidently applies to a great many transition metal chalcogen and pnigogen systems. Some kinetic data for the reactions between niobium and arsenic or antimony, respectively, have been collected. The same experimental technique was used, but with atomic ratios Nb:X
Metals,
7 (1964)
227-230
229
SHORT COMMUNICATIONS
measurements on each sample) of the niobium pnictide layer relative to the original thickness of the niobium sheets (in these experiments sheets of constant thickness were used) as a function of the reaction time. The results show that in the temperature range 7oo-925°C the reaction between niobium and antimony follows a linear rate law. This suggests that the antimonide scales formed under these conditions have very poor protective properties. The metallographic cross-sections which showed porous NbSbz scales with comparatively large holes are consistent with this view.
20
40
60
80
Time
(hours)
100
120
140
---w
Fig. I. The reactions between niobium and arsenic at 7zoY (0) and between niobium and antimony at 700°C (Cl), 800” (v) and 925’C (a). Relative thickness of niobium pnictide layer as a function of time.
In the same temperature interval the reaction between niobium and arsenic seems to be difficult to interpret in terms of a definite parabolic or linear rate law (see the 720°C run shown in Fig. I). The metallographic cross-sections showed that the NbAsz scales were more dense and more compact than the ~~)rrespondingNbSbz scales. The growth habits of the scales also differ in another way. The arsenide specimens showed a preferential tendency to react at the edges of the metal cores with a subsequent cleavage of scales at the edges. Micro-indentation hardness (measured with a Vickers pyramid at a single load of 42 g) recorded over transverse sections of the NbAsz scales showed a gradual increase from 1400kg/mm2 at the metal/gas interface to a constant value of 2400 kg/mm2 obtained at about 0.08 mm from the interface. Similar measurements carried out on the NbSbz scales were not reproducible in agreement with the porosity and holes in these scales. The micro-indentation hardness (MHs, = 770 kg/mm2, MHioll = 505 kg/mm” and MH2, = 340 kg/mm”) in the metal core did not depend upon the reagent (arsenic or antimony), the temperature or the duration of the reaction and was constant throughout the metal core. The pnigogen penetration into the metal core or the diffusion of metal outwards is thus rather small. As seen from Fig. I the reactions between niobium and arsenic or antimony leading to the formation of the NbX2 compounds are fairly fast. For most forms of niobium
SHORT COMMUNICATIONS
230
starting material the reactions will be completed and equilibrium obtained in some days. With regard to the preparation of NbAs, Nb&b and Nb&bd, which was one of the main objects of this study, our conclusions are: (i) These compounds cannot be prepared by a direct reaction from the elements using coarse-grained niobium. The resulting products are heterogeneous, with niobium and niobium di-pnictides as the main constituents. (ii) The compounds might be obtained from sufficiently powdered niobium. However, traces of hydrogen present in the niobium produce considerable amounts of impurities (NbO, Nb5Si3 and crystalline SiOz). (iii) NbAs, NbaSb and NbsSb4 are best prepared by thermal decomposition of NbAsz and NbSbz, both of which are easily made. According to FURUSETH AND KJEKsHus3,
NbAs
NbsSb4
Nb&b
and
is obtained by
after complete
degradation
of NbSba
degradation at 830°C
of NbAsz and
IOOO’C
at IIOO’C
and
respectively.
The authors wish to thank Professor H. HARALD~EN for his interest in this study and for placing
laboratory
Kjemisk Institutt A, University of Oslo, Blindem (Norway)
facilities
at their disposal.
S. FURUSETH A. K JEKSHUS
1 S. FURUSETH AND A. KJEKSHUS, Nature, in press. 2 S. FURUSETH AND A. KJEKSHUS, Acta Cryst., in press. 3 S. FURUSETH AND A. KJEKSHUS, Acta Chem. &and., in press. 4 S. FURUSETH AND A. KJEKSHUS, Acta Cryst., in press. 5 H. BOLLER AND E. PARTHB, Acta Cryst., 16 (1963) 1095. 6 B. T. MATTHIAS, E. A. WOOD, E. CORENZWIT AND V. B. BALA, J. Phys.Chem. Solids, I (1956) 188. 7 E. A. WOOD, V. B. COMPTON, B. T. MATTHIAS AND E. CORENZWIT, Acta Cryst., II (1958) 604. 8 M. V. NEVITT, Trans. AIME, 212 (1958) 350. 8 H. SCHAFER AND K.-D. DOHMANN, Z. Anorg. ALlgem. Chem., 2gg (1959) 197. 10 A. KJEKSHUS, Acta Chem. Scand., 15 (1961) 159. 11 G. S. SAINI, L. D. CALVERT AND J. B. TAYLOR, Can. J. Chem., 42 (1964) 630.
Received March Irth, 1964 J. Less-Common Metals, 7 (1964) 227-230