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ANOMALIES IN ELECTRICAL RESISTIVITY OF VANADIUM NJTR~DE
Anomalies in the Electrical Resistivity of Vanadium Nitride
by L. Glasser and J. Hoy Department of Chemistry, University of the Witwatersrand, Johannesburg, South Africa
(Received April $1, 1966)
The variation of the electrical resistivity of vanadium nitride with temperature has been studied. Near room temperature, where the specific resistivity is about 100 pohm-cm., there is an approximately linear increase of resistivity with temperature, but the resistivity falls away from the initial slope a t higher temperatures (around 4.50"))sometimes rising linearly once again after reaching a minimum. The origin of the anomaly is discussed, and oxygen impurities are suggested as its source.
Two factors contribute to an interest in the electrical resistivity of vanadium nitride, one of the refractory hard meta1s.l Firstly, the room-temperature resistivity measured by different authors varies from 85 to 330 pohm-cm.2-6 with the lower values (85-100 pohm-em.) being more commonly reported. A similar variability in titanium carbide has been attributed to uncontrolled carbon vacancies.6 The second motivation for the study is that, when vanadium nitride was used as a catalyst for the decomposition of ammonia,' a sharp discontinuity in the Arrhenius plot of the logarithm of the rate of ammonia decomposition against the reciprocal of the absolute temperature was observed a t 420". Recent work,* performed in conjunction with that here presented, shows that such changes in the catalytic activity appear regularly, although apparently a t rather higher temperatures (450-500")than first reported. Because of the wellestablished relation between catalysis and electronic proper tie^,^ it was of interest to examine the electrical resistivity.
Experimental Section Sample Preparation. Vanadium nitride was preparedlo by heating Riedel de HaEn analyzed grade ammonium vanadate (99.9% pure) for 24 hr. at l l O O o in unglazed porcelain boats in a stream of pure (99.98%), dry ammonia. The solid was allowed to cool slowly to room temperature, ground, and then renitrided several times, until a uniformly brown powder was obtained. This VN powder was formed a t a pressure of 250 kg. into pellets, 13 mm. in diameter and 1 mm. thick. These pellets always split across the diameter
on removal from the die; the half-pellets were then sintered for several days in an atmosphere of ammonia, at temperatures up to 1100", before the electrical conductivities were measured. The upper temperature of 1100" was chosen because this is the highest temperature at which VNl.o is stable, the compound losing appreciable amounts of nitrogen at higher temperature~.~ Chemical Analysis. Vanadium in the VN was determined by boiling the powder in concentrated nitric acid to convert it to VZO5. After evaporation of the nitric acid, the VZO5was dissolved in concentrated sulfuric acid, diluted with water, and titrated amperometrically with standard ferrous solution. The vanadium content was found to average 76.7 wt. %, the theoretical vanadium content for the formula 78.4wt. %. V N I . being ~ Analysis of the nitrogen in VN was performed by the Kjeldahll' method with addition of small amounts (1) P. Schwarzkopf and R. Kieffer, "Refractory Hard Metals," The Macmillan Co., New York, N. Y., 1953. (2) S. M. L'vov, V. F. Nemchenko, and G. V. Samsonov, Dokl. Ahxzd. Nauk S S S R , 135, 577 (1960). (3) I(.Moers, 2.A w g . Allgem. C h a . , 198, 262 (1931). (4) E. Friederich and L. Sittig, ibid., 143, 293 (1925). (5) V. A. Epelbaum and B. F. Ormont, Acta Physicochim. U R S S , 22, 319 (1947). (6) W. S. Williams, Phys. Rev., 135, A505 (1964). (7) C. R. Lotz and F. Sebba, Trans. Faraday Soc., 53, 1246 (1957). (8) D.A. King and F. Sebba, J. Catalysis, 4, 430 (1965). (9) G. M. Schwab, Angew. Chem. Intern. Ed. Engl., 2, 59 (1963). (10) V. A. Epelbaum and A. K. Brager, Acta Physicochim. U R S S , 13, 595 (1940). (11) V. A. Epelbaum and B. F. Ormont, Zavodsk. Lab., 14, 104 (1948).
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of HgCIP to the KzS04 catalyst. Complete digestion is effected in 15-20 min., and fairly reproducible nitrogen values were obtained in this way. The nitrogen content of VN was estimated at 19.0-19.5 wt. %, the theoretical nitrogen content for the formula VNl.,, being 21.6 wt. %. Electrical Measumments. The conductivity cell, based on one described by Butler,13 utilized four goldfoil electrodes, one pair serving as current contacts and the other as potential probes. The current contacts were connected in series with a four-terminal 0.1-ohm standard resistance and a 12-v. heavy-duty battery. The potentials developed across both the pellet and the standard resistance during current flow were measured by means of a Pye vernier potentiometer having a smallest division of 1pv. Before resistancetemperature measurements were commenced, the furnace containing the pellet and electrode assembly was flushed with the gas to be used subsequently in the experiment, viz., argon, nitrogen, or ammonia, for at least 12 hr. At least two readings were taken for each resistance measurement, with opposite directions of current flow to minimize the effects of thermal gradients. The specific resistivities of the specimens were determined by comparison with the resistance of an iron pellet of similar dimensions under the same conditions (but sintered in hydrogen), giving a specific resistivity for VN of the order of 100 pohm-cm. a,t room temperature.
Results The resistance-temperature characteristics of many vanadium nitride samples were carefully observed from room temperature up to about 550". Anomalous results were observed, generally in the temperature range 350-500", independent of the ambient atmosphere, viz., argon, nitrogen, or ammonia. Any changes in nitrogen content of the solid during the experiments were too small to cause observable changes in the conduction behavior, as shown by the fact that the resistance-temperature curve for any one sample was reproducible. The form of the anomaly and the temperature a t which this occurs are not always reproduced in different samples. The resistance-temperature curves may be classified broadly into three closely related types. Type I curves exhibit normal metallic behavior, i.e., an almost linear increase of resistance with temperature, up to about 250"; above this temperature the resistance deviates from the initial curve and decreases with increasing temperature over a narrow range, thereafter rising again. Type I1 curves are similar except that no rise is observed after the maximum. The Journal of Physical Chemistry
1
I 0
a
100
I
200 300 400 Temperature, OC.
500
600
Figure 1. The three types of resistance-temperature behavior for VN: 0, I; X, 11; 0,111. Resistances for each sample are shown relative to their own room-temperature resistance.
Type I11 curves merely show an alteration in slope at certain temperatures, without any decrease in resistance. Typical curves of these three types are shown in Figure 1. The resistance behavior of any one sample observed during heating was approximately reproduced on reheating. This reversibility implies that no structural changes of significance were brought about by raising the temperature of the samples, which had all previously been sintered at a much higher temperature. No signifcant drifts of the sample resisb ances were observed during the time required to determine a full resistancetemperature curve (about 10 hr.) -8 The resistance anomalies described here may be compared with the catalytic activity anomalies of VN reported in the literature.'J
Discussion A significant property of the hard metals is their tendency to nonstoichiometry. Chemical analysis showed approximately 76.7% V and 19.3% N for the experimental material, giving a stoichiometric formula (12) R. A. Mott and H . C. Wilkinson, Fuel, 37, 151 (1958). (13) J. D. Butler, Tram. Faraday SOC.,56, 1842 (1960).
ANOMALIES IN ELECTRICAL RESISTIVITYOF VANADIUM NITRIDE
VNO.~& where X represents impurity which may be either electronically active or inert. About 4 wt Sr, of the material occurs as impurity. Spectrographic analysis showed the absence of heayymetal impurities in significant quantities, the largest proportion of impurity being Mg and Si, each estimated at less than 1%). It is reasonable to assume that a major part of the impurity is oxygen, on the basis of the work of Epelbaum and Ormont6 and of Hardy and Hulm,14who showed, by vacuumfusionanalysis, that VN prepared according to the present method is contaminated by fair proportions of oxygen. Similar resistance anomalies have been observed in compressed powder systems undergoing sintering, l6 but this explanation must be excluded in the present instance where the anomalies are observed to be reversible and independent of the temperature of sintering, above sintering temperatures of about 800”. The observation of the resistance anomalies in samples exposed to various atmospheres would appear to exclude surface effects as the source of the anomalies, which are believed to arise from bulk behavior. There are three acceptable proposals for the source of the anomalies: deficiency of nitrogen, oxygen impurity, and impurities other than oxygen. I n fact, impurities other than oxygen are unlikely to be the cause of the anomalies since results obtained using BDH AnalaR ammonium vanadate as starting material were substantially the same as those using the Riedel de Haen material despite a slightly different impurity content. The effects of nitrogen deficiency may be illustrated by reference to work on another hard metal. Eckstein and FormanlB have shown that nonstoichiometric TaC exhibits a linear relationship between resistivity and temperature, the effect of the nonstoichiometry apparently being to give rise to a residual resistivity6 with the defects scattering the electrons. The effects of oxygen on TiN have been shown by Munster“; whereas a pure stoichiometric sample of TiN displays a linear increase in resistance with increase of temperature, the resistance of oxygen-contaminated TiN falls with rise in temperature at elevated temperature, a result supported by Samsonov, et a1.18 I n view of the above-mentioned observations and the similarity in electronic structure between TIN and VN,19-21it is suggested that the present anomalous results are also ascribable to oxygen impurity whena fall of resistance at high temperatures is caused by the excitation of electrons from electronically active oxygen impurity abms into the band at these temperatures. The type 111 curves, with O&
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slight deviation from the normal “metallic” behavior, indicate that there was only a small quantity of electronically active oxygen in the samples which gave such curves. On the other hand, the samples which produce type I curves would appear to contain a moderate amount of electronically active oxygen which has the effect of causing a drop in resistance, but at the (‘trough” the oxygen atoms become exhausted of available electrons, and, therefore, the resistance commences to rise once more owing to the interaction of the conduction electrons with the phonons and defects of the VN lattice. Analysis of the type I curve given in Figure 1 shows that the decrease in resistance follows an Arrhenius-type law, with an excitation energy of about 0.4 e.v. The type I1 curves seem best described as belonging to samples for which a high excitation energy of the electrons is required. However, since the limitation on the temperature of the experiments prevented further investigation of these samples, this must be regarded as unsubstantiated. The irreproducibility of the anomalies from sample to sample may be ascribed to uncontrolled variations in the oxygen contents of the samples. Attempts have been made to detect differences in oxygen content among samples which exhibit types I, 11, and I11 behavior, but these were unsuccessful since the present method of nitrogen analysis is beset by problems such as incomplete digestion and loss of nitrogen during digestion. It is not to be expected that heat treatment in the temperature range in which VN is stable (ca. 1100”) will be s a c i e n t to drive off all oxygen. Thus, the oxygen will not have been removed by sintering. It must be emphasized that the results described here are consistent with any activated process, and their source in oxygen impurity is conjectural insofar as it has not yet been found possible to control oxygen impurity, nor even to determine oxygen content reliably; attempts to prepare VN uncontaminated with oxygen, by nitriding pure vanadium metal, were unsuccessful. (14) G. F. Hardy and J. K. Hulm, Phys. Rev., 93, 1004 (1954). (15) W. Trsebiatowski, 2.Anorg. Allgem., Chem., 198, 262 (1931). (16) B. H. Eckstein and R. Forman, J. Appl. Phys., 33, 82 (1962). (17) A. Munster, Angew. Chem., 69, 281 (1957). (18) G. V. Samsonov, T. 8. Verkhoglyadova, S. M. L’vov, and V. F. Nemchenko, DOH.Akad. Nauk SSSR, 142, 862 (1962). (19) H. Bils,Z. Physik, 153, 338 (1958). (20) S. P. Denker, Phys. Chem. Solids, 25, 1397 (1964). (21) V. Ern and A. c. Switendick, Technical Report 192, Laboratory for Insulation Research, Massachusetts Institute of Technology, Cambridge, M ~ S S .Oct. , 1964.
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Acknowledgments. The authors wish to thank the Director and Dr. C. Carlton of the Government
Metallurgical Laboratory, Johannesburg, for the spectroscopic analyses.
NOTES
Photolysis of 4-Amino-P'-nitroazobenzene in Dimethylformamide
by J. H. DeLap, H. H. Dearman, D e p a r t m t of Chemistry, University of North Carolina, Chapel Hill,.North Carolina
and W. C. Neely Chemtrand Research Center, Inc., Durham, North Carolina (Received April 29, 1966)
The mechanism of Cis-trans isomerization in aromatic azo compounds has received considerable attention in recent y e a r ~ . l - ~However, little emphasis has been placed on the irreversible photochemical reactions of these compounds. Blaisdel14 has concluded that the photolysis of azobenzene and of 4-amino-4'-nitroa5obenzene (ANAB) in alcohol and hydrocarbon solvents involves hydrogen abstraction from the solvent by an excited state of the azo compound. Following this initial step, cleavage of the hydrazobemene intermediate leads to the formation of anilines. We have studied the wave length dependence of the photolysis of ANAB in dimethylformamide (DMF) and also have examined the possibility of triplet state participation in the photochemical process. ANAB was purified from a commercial sample by repeated crystallizations from water-acetone mixtures followed by chromatography on an alumina column with methanol as the eluent. The purified material gave a single spot with paper chromatography. Degassed samples approximately lov5 M were exposed in silica cells to the radiation from a 150-w. xenon lamp which produced a flux of 5.2 X 10l6 quanta sec.-1. Irradiation with discrete wave length regions was accomplished by the use of a series of glass cut-off filters in conjunction with an ultraviolet transmission filter (Corning 7-54). The Journal
of
Physical Chemistry
The absorption spectrum of ANAB in isooctane and in DMF is shown in Figure 1. The n-r* transition, easily discernible in the spectrum of azobenzene, is obscured by the intense r a * transition in the substituted molecule6 a t 4600 8. in DMF. Light with wave length in the region of these transitions is found to be completely ineffective in producing photolysis of ANAB in DMF since the reaction proceeds with the same rate behind the ultravioletotransmission filter (transmitting from 2275 to 4100 A.) as with unfiltered light. In fact, exposure behind sharp cut-off filters shows that wave lengths less than 2560 A. are necessary for photolysis. The spectral changes accompanying photolysis are shown in Figure 2. Light of the photochemically active wave lengths is almost completely absorbed by the solvent. We can obtain a rough estimate of the partitioning of available light between ANAB and DMF by measuring the extinction coefficients of the components individually in a nonabsorbing medium such as isooctane. By such a procedure and with the assumption that no significant change in absorbance a t the active wave length occurs in the formation of the photolysis solutions, the ratio of uanta absorbed by DMF to that by ANAB a t 2550 . is found to be 1.5 X lo6and increasetj a t shorter wave length. We have determked that approximately 2% of the light flux is active in photolysis. Thus, the upper limit of the rate of direct photolysis is 7 X M see.-' which is to be compared with the observed rate of 2 X lo+ M sec.-l. The photochemical inactivity of the strong visible absorption band indicates that the lowest singlet states of ANAB are not participating in the reaction. This result does not rule out photochemical reactivity of a
B
(1) 8. Yamashita, H.Ono, and 0. Toyama, Bull. Chem. Soc. Japan, 35, 1849 (1962). (2) 8. Malkin and E. Fischer, J . Phys. C h m . , 66, 2482 (1962). (3) R. H.Dyck and D. 8. McClure, J . C h m . Phys., 36,2326 (1962). (4) B. E. Blaisdell, J . SOC.Dyers Colourists, 65, 618 (1949). (6) M. B. Robin and W. T. Simpson, J. Chem. Phys., 36,680 (1962).
