CONDUCTIVITY OF NiO-a-Fe203 SYSTEM
1095
Electrical Conductivity of the Nickel Oxide-a-Ferric Oxide System by Jae Shi Choi and Ki Hyun Yoon Department of Chemistry, Yonsei University, Seoul, Korea
(Received August
4,1969)
The electrical conductivity of polycrystalline samples of the NiO-a-FezO3 system containing 2.1, 4.2, 8.2, 10, 19.2, 34.8, and 41.6 mol % of NiO has been measured in the temperature range of 400 to 1100" under oxygen to 152 mm, using a new contact method which has been devised in this laboratory. pressures from 1.12 X It was found that, for a given temperature, the slope of log conductivity us. mole per cent of NiO is negative up to a certain mole per cent of added NiO, becoming positive for further increase in NiO concentration. This inversion point was found to move toward higher values of NiO concentration at higher temperatures. This is explained according to the controlled valency principle. The variation of conductivity with temperature is described. Fermi degeneracy is found to occur above about 34.8 mol % of the added NiO.
Introduction A quantitative investigation of the electrical conductivity of NiO-a-Fez03 system has not been published since 1960, when Lord and Parker1 reported on the electrical resistivity of nickel ferrite. Their experimental results were limited to the investigation of the influence of oxygen pressures on the electrical properties of this material. This investigation has been undertaken to determine the inversion point (decrease to increase) of the electrical conductivity of NiO-a-Fez03 as a function of the amount of added NiO by analyzing the results of conductivity measurements made over a range of oxygen partial pressures on samples of varying relative composition. It is generally believed that the conductivity of a semiconducting oxide can be shifted in the direction of increased P conductivity or decreased N conductivity by doping small proportions of lower valency cations and that the reverse is true for higher valency cation doping. Because the only necessary conditions for the doping effect, are solubility in the solid state and a similarity of cation radii, the electrical properties of the NiO-a-Fez03 system seem to be similar to that of Zn02 doped &O. The electrical conductivity of the NiOa-Fet03 system depends on temperature and the ambient oxygen pressure in the sintering process. I n these measurements, thermal hysteresis was negligible. Since electrical conductivity has been correlated with defect structure, the influence of oxygen pressure of the sintering process in the preparation of the spinel NiOa-FezOa system is important. With high oxygen pressure during sintering, the NiO-a-Fen03 system turns to a P type semiconductor and under low oxygen pressure it converts to an N type.l The explanation of the conduction mechanism is given by applying the principle of controlled ~alency.~-e
Sample Preparation Specpure NiO powder from Johson Matthey Go. and a-Fe203, which was prepared from chemically pure
Feel2 and KOH by the wet method according to the procedure of Balz,' were used for the preparation of NiO-a-Fe203. The sample was identified as a-Fe20s by X-ray diffraction. NiO and a-Fe203were weighed precisely, mixed in varying proportions, ball-milled for 15 hr in a C2H,OH solution, and then dried at 150". Three grams of the powder mixture was made into a pellet containing four Pt leads (length 10 mm, radius 0.04 mm) with 0.6 g of powder between leads under a pressure of 2 tons/cm2. This pellet was presintered for 3 hr a t 800" and then sintered for 3 hr at 1200" under oxygen pressure and then cooled rapidly. A difficult problem in the measurement of electrical conductivity is the contact method. In this l a b o m tory, the 4 Pt leads were inserted into the NiO-a-Fez03 powder at equal intervals and the combination compressed under high pressure and sintered to allow interdiffusion of both cations. By this method, contact resistance is kept constant with temperature change and good contact between the four Pt leads and sample is maintained.
Experimental Section The experimental apparatus used is shown in Figure 1. The vacuum system was connected to a Cenco Hyvac force pump through an EC GF-20A oil diffusion pump. The pressure was measurable down to mm. Among the four Pt leads which were in contact with the sample, the two inside leads were connected to a battery through an ammeter. Before the sample was inserted, it was first polished with abrasive (1) H. Lord and R.Parker, Nature, 188,929(1960). (2) K.Hauffe and A. L. Vierk, 2.Phys. Chem., 196,160 (1950). (3) W.Schottky, ibid., 1329,335 (1935). (4) C.Wagner, and W.Schottky, ibid., B11, 163 (1930). (5) E.J. W.Verwey, P. W. Haaijman, F. C. Romeijn, and C. W. Van Oosterhout, Philips Research Repts., 5 , 173 (1950). (6) G.Brouwer, ibid., 9, 366 (1954). (7) W. Balz, Badische Anilin and Soda Fabric., Fr. 1, 357, 866
(1964). Volume 74, Number 6 March 6 , 1970
1096
JAESHI CHOIAND KI HYUNYOON IOOO’C 900°C 800°C 7OO0C I
Figure 1. Schematic drawing of the furnace assembly and vacuum system: 1,2, rotary pumps; 3, diffusion pump; 4,5, Dry Ice traps; 6, McLeod gauge; 7, quartz tube; 8, Vycor tube; 9, Pt lead; 10, NiO-a-FezOs sample; 11, furnace; 12, PtrRh thermocouple; 13, ammeter; 14, manometer; 15, 02 storage tank; 16, CaCl2 tube; 17, KClOs flask.
paper of silicon carbide, etched in dilute HNOa, washed with distilled water, dried, and connected to the Pt leads of the sample container. When the temperature and the oxygen pressure were adjusted to the desired values, the stopcock which connected the vacuum system and sample container was closed. After 1 hr of annealing,* the temperature was raised by increments of 20”. The dc current in the sample was measured with a Hewlett-Packard de microvolt ammeter and the dc voltage was measured with a Leeds & Northrup K-2 potentiometer. The galvanometer used was a Leeds & Northrup Type E galvanometer whose sensitivity is 0.005 pA/mm. The current in the sample was kept below 1.5 mA.9 Typical analyses of these samples show that they contain 40 ppm, mainly Cu, Si, Al, and Pb. Experimental Results The results of the electrical conductivity measurements are shown in Figure 2. Log conductivity for seven different samples is plotted vs. the reciprocal of absolute temperature. We may note that for every curve a straight line is obtained. The slope is negative and its value depends on the concentration of N O . From samples 1 to 4 the slope becomes more negative, whiIe at higher concentrations of X i 0 the curves become almost flat, showing that the temperature dependence becomes less for higher concentrations of NiO. The electrical conductivity isotherms of various compositions in NiO-a-FezOs systems are presented in Figure 3. The clusters of points show conductivity values a t the same temperature under the different oxygen pressures for the same sample. Figure 4 shows the results of isobarics in which log Q is plotted against 1/T. At high temperatures there is little variation in the value of log Q over the whole pressure range. As seen already in Figure 4, noting points falling on the same position indicate that the The Journal of Physical Chemistry
I
1
-
-
4-
5OOOC
I
mole mole mole mole mole
% %
Ni034.8
% mole %
NiO 41.6
mole
%
NiO 2.1 d--N i O 4.2 NiO 8.2 -Y-dN i O 10.0 -0NiC 19.2
600OC
I
I
Oh %
1
0.8
0.9
1.0
1.1 b
1.2
1.3
1.4
1.5
1000 / T
Figure 2. Representative temperature dependence of conductivity of composition in the system Ni0-a-Fe2O8.
conductivity is nearly independent of the ambient oxygen pressure. Discussion Since the sintering was done under low oxygen pressure (152 mm) the excess Nia+ ions, which were dissolved in the NiO-cr-Fe203 lattice, were reduced completely to Ni2+ions and the electrons which were released from the oxygen reduced the Fe8+ ions to Fez+ ions; Fe3+ e --+ Fe2+. This may be represented by1
+
NiO
+ a-Fe203 -+
+
Ni2+Fea+2-2sFe2+2s02-4--S1/2S
(02)
Thus the electrons function as the carriers. Also thermoelectric power showed that all samples had negative Seebeck coefficients. Therefore, as shown in Figure 4, the electrical conductivity is not affected by the partial oxygen pressure regardless of the doped NiO mole percentage. As shown in Figure 3 the increase in the amount of the doped NiO, according to the principle of controlled valency, prevents the process Fe*+ e -+ Fez+ conduction mechanism of the NiO-a-FezOs system. Because the number of conduc-
+
R.S. Toth, R. Kilkson, and D. Trivich, J . A p p l . Phys., 31,1117 (1960). (9) W.J. Moore, “Physical Chemistry,” Prentice-Hall, Inc., Englewood Cliffs, N. J., 1962. (8)
1097
CONDUCTIVITY OF NiO-a-Fez03 SYSTEM
O t
1
-2
I
1
I
I
I
I
I
2
4
8
10
20
30
40
L
NiO
mole
%
Figure 3. Values of conductivity of NiO-a-FezO3 as the function of the amount of NiO added. Clusters of points show conductivity values measured a t different oxygen pressure for the same sample.
tion electrons is decreased, the electrical conductivity is decreased. I n the NiO-a-FezOa system, when the amount of doped NiO is increased to more than about 10 mol %, the conductivity increases due to the in600OC
900°C 800°C 700% I
I
I
400'C
SOO'C
I
I
OXYGEN
I
P R E S S U R E LmmHg) ..-x-.-
1.1214X I O - 5 3.1 I 5 0 x 10-5
-0-
6.1054x
-h-
7.7876x 5 2644 x IO+
-A-
-t
152
creasing positive holes arising from Ni3+ions dissolved in the NiO-a-FezOa lattice through an excess amount of nickel oxide. I n other words, the concentration carrier in the NiO concentration region then mostly converts to positive holes due to the excess NiO. As a result, the electrical conductivity increases due to the increasing doped nickel oxide in the NiO-a-FezOs system. With increasing temperature, independent of oxygen pressure, the inversion point (decrease to increase) of the electrical conductivity shows a tendency to move in the direction of increasing mole percentage of nickel oxide. The reason, in general, is that when a-FezOais in its pure state, it begins to reduce. Fe3+ e --F Fez+,a t 1388';'O but in the case of the ferrite, it reduces a t a rather lower temperature and the amount of Fez+ increases in proportion to the increases of temperature. Therefore the initial slope of conductivity vs. NiO concentration is shallower a t high temperatures (cf. Figure 3). Furthermore] according to the controlled valency principle, more electrons occur by the process Fe3+ e + Fez+ as the temperature increases, requiring additional Ni2+ ions, so the inversion point moves to the right. The decrease of conductivity above around 34.8 mol % of added NiO is not clear. In Figure 2, sample 1 shows that the inversion point appears around 600'. Above this temperature range the intrinsic conductivity of a-FezO3 appears rather than the impurity (NiO) effect. Below this temperature
+
+
0.8
0.9
1.0
1.1
1.2 L
1.3
1.4
1.5
1000 / T
Figure 4. Conductivity isobarics Of NiO-cu-Fe@s as a function of 103/T. Sample: 10 mol % ' of NiO doped a-FezOs.
(io) R.G . Richard and J. White, Trans. Brit. Ceram. Soc., 53, 233 (1964).
Volume 74, Number 6 March 6,1970
1098
BARNEY L. BALESAND LARRYKEVAN
range it becomes an extrinsic semiconductor because of the doped NiO. Therefore it does not satisfy the general semiconductor equation, cr = Ae-E/RT,l 1 but shows a higher electrical conductivity. Sample 6 and 7, which show little change in conductivity over the temperature range 400-900”, show Fermi degeneracy because of a very high concentration of nickel oxide and conduction electrons in the system of NiO-a-FezOa. Above 900” the carrier concentration is constant because of the complete ionization of the donors; on the other hand the mobility of the carriers decreases with rising temperature due to the “impurity scattering”12 because of the excess nickel oxide. Therefore the electrical conductivity is reduced.
The values of the electrical conductivity measured both as the temperature was raised and lowered are similar. Therefore the sample was in a state of thermal equilibrium a t the moment of measurement. It appears that the new contact method devised in this laboratory is satisfactory.
Acknowledgment. The authors are grateful to the Graduate School of Yonsei University and Ministry of Science and Technology of Korea for support of the experimental work. (11) L.G.Uitert, J . Chem. Phys., 23,1883 (1955). (12) N. B. Hannay, “Semiconductors,” Reinhold Publishing Corp. New York, N . Y., 1959.
Electron Paramagnetic Resonance Studies of Silver Atom Formation and Enhancement by Fluoride Ions in ?-Irradiated Frozen Silver Nitrate Solutions by Barney L. Bales’ and Larry Kevan2 Department of Physics and Department of Chemistry, University of Kansas, Lawrence, Kansas (Received July 14,1909)
060.44
The y radiolysis of AgNOa ices a t 77°K produces trapped Ago, OH, and NOz. Addition of fluoride ion increases Ago, prevents NO2 formation, increases the total number of observable spins, and increases the linear range of the dose-yield curve for Ago. The initial Ago yields are G(Ago) = 1.2 in 1M AgNOs and G(Ag0) = 3.2 in 1 M AgNOa-0.5 M KF. These effects indicate that fluoride ion acts as an efficient hole trap for HzO+and prevents electron-hole recombination. Low fluoride concentrations are eff eotive; thus some HzO+ is mobile.
Introduction The radiolysis of ice has been the subject of much researcha and is understood in terms of the initial reaction scheme given in (1) and (2). The details of the
+ OH HzO+ + em-
€ 1 2 0 --+ H
H20 --+
(1.)
(2)
fate of the electron and the hoie depend on the nature of the solutes present in the ice and, to some extent, on the phase of the ice.“ Shields6 and Zhitnikov and Orbelie have shown that 7-irradiated silver salt solutions at 77°K yield silver atoms which were attributedb~~ to the electron-capture reaction em-
+ Ag+ +AgO
(3)
The epr spectra of Ago were analyzed, but no quantitaThe Journal of Physical Chemistry
tive data on yields or on the reactions occurring in the frozen system were reported. One interesting observation was that fluoride ion enhanced the yield of A@ in the irradiated silver salt ices. Shields6 inferred that fluoride ion “promotes” the reactivity of electrons with (1) Department of Physics. (2) Department of Chemistry. Address inquiries to this author at Department of Chemistry, Wayne State University, Detroit, Mich.
48202. (3) L. Kevan in “Radiation Chemistry of Aqueous Systems,” G. Stein, Ed., Interscience Division, John Wiley & Sons, Inc., New York, N. Y., 1968,pp 21-72. (4) H. Hase and L. Kevan, J.Phys. C ~ E W 73,3290 Z., (1969). (5) L.Shields, J . Chem. Phys., 44, 1685 (1966); Trans. Faraday soc., 62, 1042 (1966); I,, Shields and M. C. R. Symons, Mol. Phys., 11, 57 (1966). (6) R. A. Zhitnikov and A. L. Orbeli, Fiz. Tverd. Tela., 7,1929 (1905); Sou. Phys. Solid State, 7,1559 (1960). (7) B.L. Bales and L. Kevan, Chem. Phys. Lett., 3,484 (1969).
