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Thermionic emission from CaO in Ba vapor
SURFACE
SCIENCE 29 (1972) 291-296 o North-Holland
LETTERS
THERMIONIC
TO THE
EMISSION
Received 24 September
Publishing Co.
EDITOR
FROM
1971; revised manuscript
CaO IN Ba VAPOR
received 19 November
1971
Many materials undergo a large reduction in work function when coated with approximately a monolayer of bariuml). This is true of most metals and of at least some nonmetallic materials including members of the alkaline earth oxide family29s). Calcium oxide is an insulator with a band gap near 7 eV495) and when heated invacuum, it is found to be n-type with a work function in the range 2 to 3.5 eV 2, 6). The partial pressure of Ba over CaO necessary to maintain an optimum coverage at a given temperature is not known. In order to study the effect of Ba upon the thermionic emission from CaO for a range of CaO temperatures and a range of Ba pressures an experimental tube was built with a CaO cathode and a Ba reservoir as shown in fig. 1. The envelope was made of titanium metal and forsterite ceramic.
Fig. 1.
Schematic drawing of the experimental 291
tube.
292
H. H. GLASCOCK,
JR.
Iron-titanium and nickel-titanium brazes were used7). The heater was bonded to the back of the cathode disk in a separate chamber from the cathode emitting surface. The tungsten cathode substrate was painted with a wet coating of calcium carbonate. The surface density of the oxide equivalent of this carbonate coating was 0.3 mg/cm2. The equivalent thickness of such an oxide coating of theoretical density is 1 urn. The calcium carbonate had been ball milled in an aluminum oxide container for 90 hr. Amy1 acetate was used as the liquid with a small amount of nitrocellulose added for binder. Conversion from calcium carbonate to calcium oxide occurred during the final braze in an ion pumped system while the cathode was held above 800°C for 10 min with a maximum temperature near 1000°C. The temperature of the tube envelope was controlled by an electric oven while the reservoir was held at a lower temperature by a second oven, These temperatures were measured using chromel-alumel thermocouples. A Pt versus Pt-10 Rh thermocouple affixed to the back of the cathode substrate was used to obtain a calibration curve of heater power versus cathode temperature for an envelope temperature of 297°K and a fourth power dependence was found to exist over most of the temperature region of interest. The cathode temperature was subsequently determined from the heater power and the envelope temperature using the generalized StefanBoltzmann laws). Measurements were also made with the cathode heated exclusively by the oven as a check on the above procedure. Barium reservoir temperatures were not converted to barium vapor pressures since reliable barium vapor pressure versus temperature data is only available at the highest temperatures usedg). The maximum reservoir temperature shown on plots here, 790”K, corresponds to a barium vapor pressure of about 5 x IO-’ Torrra) or a barium flux of 5 x lOi atoms/cm2 sec. Electron emission was measured with cathode temperatures between 700 and 1100°K and barium reservoir temperatures between 297 and 800°K. The voltage between the cathode and the anode was varied between 0 and 50 V, and the emission current and the applied voltage were recorded. These dc measurements were limited to current densities below 0.1 A/cm2 since at higher current density levels, reversible time dependent changes in the cathode emission occurred. The cathode work functions reported here are “zero field” values obtained from Schottky plotsrl). A representative set of Schottky lines is shown in fig. 2. Shown on each plot is the zero-field emission current density, J,,, the corresponding effective work function12), 40, the temperature of the CaO, T(CaO), and the temperature of the Ba reservoir, T(Ba). The Schottky spacing, d, is obtained from the Schottky equation, and for this set of
THERMIONICEMISSIONFROM
10-4
0
I
2 Fig. 2.
3
” l/2
CaO
IN
4
Ba
293
VAPOR
5
6
7
Schottky plots.
Schottky lines is 2.5 x 10e3 cm. This is about + the actual spacing between the cathode and the anode. Such small Schottky spacings are usually observed with alkaline earth oxide cathodesra,r4). Fig. 3 shows cathode effective work function versus cathode temperature for various barium reservoir temperatures. These curves are independent of the direction of the temperature change. For sufficiently high cathode and reservoir temperatures the curves exhibit a single maximum in work function. One curve, T(Ba)=752”K, was measured without heater current as the cathode temperature was fixed by the oven temperature. A maximum in work function does not occur if Ba is deposited upon a metal. The observed maximum when barium is deposited upon the wide band-gap insulator calcium oxide is probably related to a process occurring at re-
294
H. H. GLASCOCK,
JR.
r
2 .2 -
2 ,I -
0,
2. 0 t
I.9-
I.8-
700 Fig. 3.
Variation of work function with cathode temperature reservoir temperature.
for fixed
latively high values of T(Ba) and T(Ca0) involving a shift in the Fermi energy. A shift may be caused by the destruction or creation of acceptor sites or donor sites. The nature and behavior of donors and acceptors in oxide cathodes is still a matter of controversy after many years of studyr5,16). In fig. 4 is shown the cathode work function versus barium reservoir temperature for various cathode temperatures ranging from 750 to 950°K. Each curve exhibits a work function minimum corresponding to an optimum equilibrium coverage of barium. Similar behavior is observed when barium is deposited upon most metals. The minimum shifts to higher reservoir temperatures (higher barium pressure) as the cathode temperature is increased. Beyner and Nikonova) made emission measurements from a CaO cathode
THERMIONICEMISSIONFROM
I.71 300
Fig. 4.
I
400
CaO
Ba
29.5
VAPOR
I
I
600 'K
700
I
500 TM
IN
Variation of work function with reservoir temperature cathode temperatures.
J
800
for fixed
in a flux of Ba atoms from an evaporator. They found that an optimum coverage of Ba on CaO resulted in a work function of 4 = 1.32 +4.5 x 10v4 T for 600~ T(Ca0) < 800°K. Over the small temperature interval, where a comparison can be made, 750°K to 800”K, their minimum work functions are 0.1 eV lower than those reported here. It is not clear from their paper that a correction has been made for field effects; if not, such a correction would result in even better agreement. H. H. GLASCOCK, JR.
General Electric Corporate Research and Development, Schenectady, New York 12301, U.S.A.
References 1) V. S. Fomenko, in: Handbook of Thermionic Properties, Ed. G. V. Samsonov (Plenum Press Data Div., New York, 1966) pp. 118-125. 2) K. S. Beyner and B. P. Nikonov, Radio Eng. Electron. Phys. (USSR) (EnglishTransl.) 10 (1965) 408.
296
H. H. GLASCOCK,
JR.
3) J. E. Beggs, General Electric Res. and Devel. Center, private communication (April 1967). 4) H. H. Glascock, Jr. and E. B. Hensley, Phys. Rev. 131 (1963) 649. 5) R. C. Whited and W. C. Walker, Phys. Rev. 188 (1969) 1380. 6) B. J. Hopkins and F. A. Vick, Brit. J. Appl. Phys. 9 (1958) 257. 7) J. E. Beggs, Inst. Radio Engrs. Trans. Electron Devices 3 (1956) 93. 8) S. S. Kutateladze and V. M. Borishanskii, A Concise Encyclopedia of Heat Transfer (Pergamon, New York, 1966) pp. 256-257. 9) A, N. Nesmeyanov, in: Vapor Pressure of the Chemical Elements, Ed. R. Gray (Elsevier, Amsterdam, 1963) pp. 186-189. 10) E. Hinnov and W. Ohlendorf, J. Chem. Phys. 50 (1969) 3005. 11) E. Coomes, in: Methods of Experimental Physics, Vol. 6, Part B, Eds. K. LarkHorovitz and V. A. Johnson (Academic Press, New York, 1959) pp. 128-136. 12) E. B. Hensley, J. Appl. Phys. 32 (1961) 301. 13) D. A. Wright and J. Woods, Proc. Phys. Sot. (London) B 65 (1952) 135. 14) G. A. Haas and E. A. Coomes, Phys. Rev. 100 (1955) 640. 15) P. Zalm, in: Advances in Electronics and Electron Physics, Vol. 25, Ed. L. Marton (Academic Press, New York, 1968) pp. 213-214. 16) R. 0. Jenkins, Vacuum 19 (1969) 353.
SCIENCE 29 (1972) 291-296 o North-Holland
LETTERS
THERMIONIC
TO THE
EMISSION
Received 24 September
Publishing Co.
EDITOR
FROM
1971; revised manuscript
CaO IN Ba VAPOR
received 19 November
1971
Many materials undergo a large reduction in work function when coated with approximately a monolayer of bariuml). This is true of most metals and of at least some nonmetallic materials including members of the alkaline earth oxide family29s). Calcium oxide is an insulator with a band gap near 7 eV495) and when heated invacuum, it is found to be n-type with a work function in the range 2 to 3.5 eV 2, 6). The partial pressure of Ba over CaO necessary to maintain an optimum coverage at a given temperature is not known. In order to study the effect of Ba upon the thermionic emission from CaO for a range of CaO temperatures and a range of Ba pressures an experimental tube was built with a CaO cathode and a Ba reservoir as shown in fig. 1. The envelope was made of titanium metal and forsterite ceramic.
Fig. 1.
Schematic drawing of the experimental 291
tube.
292
H. H. GLASCOCK,
JR.
Iron-titanium and nickel-titanium brazes were used7). The heater was bonded to the back of the cathode disk in a separate chamber from the cathode emitting surface. The tungsten cathode substrate was painted with a wet coating of calcium carbonate. The surface density of the oxide equivalent of this carbonate coating was 0.3 mg/cm2. The equivalent thickness of such an oxide coating of theoretical density is 1 urn. The calcium carbonate had been ball milled in an aluminum oxide container for 90 hr. Amy1 acetate was used as the liquid with a small amount of nitrocellulose added for binder. Conversion from calcium carbonate to calcium oxide occurred during the final braze in an ion pumped system while the cathode was held above 800°C for 10 min with a maximum temperature near 1000°C. The temperature of the tube envelope was controlled by an electric oven while the reservoir was held at a lower temperature by a second oven, These temperatures were measured using chromel-alumel thermocouples. A Pt versus Pt-10 Rh thermocouple affixed to the back of the cathode substrate was used to obtain a calibration curve of heater power versus cathode temperature for an envelope temperature of 297°K and a fourth power dependence was found to exist over most of the temperature region of interest. The cathode temperature was subsequently determined from the heater power and the envelope temperature using the generalized StefanBoltzmann laws). Measurements were also made with the cathode heated exclusively by the oven as a check on the above procedure. Barium reservoir temperatures were not converted to barium vapor pressures since reliable barium vapor pressure versus temperature data is only available at the highest temperatures usedg). The maximum reservoir temperature shown on plots here, 790”K, corresponds to a barium vapor pressure of about 5 x IO-’ Torrra) or a barium flux of 5 x lOi atoms/cm2 sec. Electron emission was measured with cathode temperatures between 700 and 1100°K and barium reservoir temperatures between 297 and 800°K. The voltage between the cathode and the anode was varied between 0 and 50 V, and the emission current and the applied voltage were recorded. These dc measurements were limited to current densities below 0.1 A/cm2 since at higher current density levels, reversible time dependent changes in the cathode emission occurred. The cathode work functions reported here are “zero field” values obtained from Schottky plotsrl). A representative set of Schottky lines is shown in fig. 2. Shown on each plot is the zero-field emission current density, J,,, the corresponding effective work function12), 40, the temperature of the CaO, T(CaO), and the temperature of the Ba reservoir, T(Ba). The Schottky spacing, d, is obtained from the Schottky equation, and for this set of
THERMIONICEMISSIONFROM
10-4
0
I
2 Fig. 2.
3
” l/2
CaO
IN
4
Ba
293
VAPOR
5
6
7
Schottky plots.
Schottky lines is 2.5 x 10e3 cm. This is about + the actual spacing between the cathode and the anode. Such small Schottky spacings are usually observed with alkaline earth oxide cathodesra,r4). Fig. 3 shows cathode effective work function versus cathode temperature for various barium reservoir temperatures. These curves are independent of the direction of the temperature change. For sufficiently high cathode and reservoir temperatures the curves exhibit a single maximum in work function. One curve, T(Ba)=752”K, was measured without heater current as the cathode temperature was fixed by the oven temperature. A maximum in work function does not occur if Ba is deposited upon a metal. The observed maximum when barium is deposited upon the wide band-gap insulator calcium oxide is probably related to a process occurring at re-
294
H. H. GLASCOCK,
JR.
r
2 .2 -
2 ,I -
0,
2. 0 t
I.9-
I.8-
700 Fig. 3.
Variation of work function with cathode temperature reservoir temperature.
for fixed
latively high values of T(Ba) and T(Ca0) involving a shift in the Fermi energy. A shift may be caused by the destruction or creation of acceptor sites or donor sites. The nature and behavior of donors and acceptors in oxide cathodes is still a matter of controversy after many years of studyr5,16). In fig. 4 is shown the cathode work function versus barium reservoir temperature for various cathode temperatures ranging from 750 to 950°K. Each curve exhibits a work function minimum corresponding to an optimum equilibrium coverage of barium. Similar behavior is observed when barium is deposited upon most metals. The minimum shifts to higher reservoir temperatures (higher barium pressure) as the cathode temperature is increased. Beyner and Nikonova) made emission measurements from a CaO cathode
THERMIONICEMISSIONFROM
I.71 300
Fig. 4.
I
400
CaO
Ba
29.5
VAPOR
I
I
600 'K
700
I
500 TM
IN
Variation of work function with reservoir temperature cathode temperatures.
J
800
for fixed
in a flux of Ba atoms from an evaporator. They found that an optimum coverage of Ba on CaO resulted in a work function of 4 = 1.32 +4.5 x 10v4 T for 600~ T(Ca0) < 800°K. Over the small temperature interval, where a comparison can be made, 750°K to 800”K, their minimum work functions are 0.1 eV lower than those reported here. It is not clear from their paper that a correction has been made for field effects; if not, such a correction would result in even better agreement. H. H. GLASCOCK, JR.
General Electric Corporate Research and Development, Schenectady, New York 12301, U.S.A.
References 1) V. S. Fomenko, in: Handbook of Thermionic Properties, Ed. G. V. Samsonov (Plenum Press Data Div., New York, 1966) pp. 118-125. 2) K. S. Beyner and B. P. Nikonov, Radio Eng. Electron. Phys. (USSR) (EnglishTransl.) 10 (1965) 408.
296
H. H. GLASCOCK,
JR.
3) J. E. Beggs, General Electric Res. and Devel. Center, private communication (April 1967). 4) H. H. Glascock, Jr. and E. B. Hensley, Phys. Rev. 131 (1963) 649. 5) R. C. Whited and W. C. Walker, Phys. Rev. 188 (1969) 1380. 6) B. J. Hopkins and F. A. Vick, Brit. J. Appl. Phys. 9 (1958) 257. 7) J. E. Beggs, Inst. Radio Engrs. Trans. Electron Devices 3 (1956) 93. 8) S. S. Kutateladze and V. M. Borishanskii, A Concise Encyclopedia of Heat Transfer (Pergamon, New York, 1966) pp. 256-257. 9) A, N. Nesmeyanov, in: Vapor Pressure of the Chemical Elements, Ed. R. Gray (Elsevier, Amsterdam, 1963) pp. 186-189. 10) E. Hinnov and W. Ohlendorf, J. Chem. Phys. 50 (1969) 3005. 11) E. Coomes, in: Methods of Experimental Physics, Vol. 6, Part B, Eds. K. LarkHorovitz and V. A. Johnson (Academic Press, New York, 1959) pp. 128-136. 12) E. B. Hensley, J. Appl. Phys. 32 (1961) 301. 13) D. A. Wright and J. Woods, Proc. Phys. Sot. (London) B 65 (1952) 135. 14) G. A. Haas and E. A. Coomes, Phys. Rev. 100 (1955) 640. 15) P. Zalm, in: Advances in Electronics and Electron Physics, Vol. 25, Ed. L. Marton (Academic Press, New York, 1968) pp. 213-214. 16) R. 0. Jenkins, Vacuum 19 (1969) 353.