Sc2O3-W matrix impregnated cathode with spherical grains

Article abstract of DOI:10.1016/j.jpcs.2008.03.013

Sc₂O₃-W matrix cathodes have been prepared by using a liquid-liquid doping method combined with high-temperature sintering. The microstructure and physical behavior of active substances of scandia-doped tungsten matrix and impregnated cathode has been studied by SEM and AES methods. The results show that the matrix has a homogeneous structure composed of W grains with spherical shape and superfine Sc₂O₃ particles dispersed uniformly over and among W grains. After impregnation, this Sc-type impregnated cathode has high emission capability. Space-charge-limited current density could reach 52 A/cm² at 850 °C_b. The high emission results from a Ba-Sc-O active layer with a thickness of about 80 nm, which is formed at the cathode surface during the activation period. Both the decrease of the thickness of active surface layer and the decrease of the content of Sc at the surface could lead to the degradation of current density during operation.

Full text of DOI:10.1016/j.jpcs.2008.03.013

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Journal of Physics and Chemistry of Solids 69 (2008) 2103– 2108
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Sc2O3– W matrix impregnated cathode with spherical grains
Ã
Jinshu Wang , Lili Li, Wei Liu, Yanchun Wang, Yiman Wang, Meiling Zhou
School of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Sc2O3–W matrix cathodes have been prepared by using a liquid–liquid doping method combined with
high-temperature sintering. The microstructure and physical behavior of active substances of scandia-
doped tungsten matrix and impregnated cathode has been studied by SEM and AES methods. The
results show that the matrix has a homogeneous structure composed of W grains with spherical shape
and superfine Sc2O3 particles dispersed uniformly over and among W grains. After impregnation, this
Sc-type impregnated cathode has high emission capability. Space-charge-limited current density could
reach 52 A/cm2 at 850 1Cb. The high emission results from a Ba–Sc–O active layer with a thickness of
about 80 nm, which is formed at the cathode surface during the activation period. Both the decrease of
the thickness of active surface layer and the decrease of the content of Sc at the surface could lead to the
degradation of current density during operation.
Received 28 September 2007
Received in revised form
11 March 2008
Accepted 11 March 2008
Keywords:
A. Microporous materials
A. Multilayers
B. Sol-gel growth
D. Microstructure
D. Surface properties
& 2008 Elsevier Ltd. All rights reserved.
1. Introduction
3–10% by weight. The doped tungsten oxide powders are reduced
to metallic tungsten powders in dry hydrogen atmosphere at high
temperature. The doped tungsten powders are pressed and
sintered into porous matrices of about 3 mm in diameter and
1 mm in thickness. Then, the matrices are impregnated with
Scandate-impregnated cathode aroused great attention since it
was invented 40 years ago [1,2], and since it was considered as the
next generation high-emission cathode. Several types of scandate
cathode have been developed during the last three decades [3–5].
Gartner et al. have developed the LAD top-layer scandate cathode
via laser ablation deposition and demonstrated that it can provide
J10% of 400 A/cm2 at 965 1Cb [6,7]. But until now only the
impregnated scandate cathode, in which scandia is added to the
barium–calcium–aluminate impregnants, has found practical
application in a limited number of vacuum microwave devices
[8,9]. However, it suffers problems of poor emission uniformity
and reproducibility [10].
barium–calcium–aluminates in
a
molar ratio of 4:1:1
(4BaO Á CaO ÁAl2O3) at temperatures of 1500–1700 1Cb. Finally,
the cathodes are washed with deionized water in order to remove
impregnant residues at the surface of the matrices.
The composition is analyzed by X-ray diffraction (D/MAX-3C).
The microstructure is observed with Hitachi S-3500N scanning
electron microscopy, equipped with Oxford Inca EDX equipment.
‘‘In situ’’ AES analyses are carried out with a VG MICROLAB MK-II
system.
Emission properties of the cathodes are tested in a close-
spaced diode configuration in either an ultra high vacuum system
with a Mo-anode or in sealed testing tubes with a water-cooled
copper anode by a computer-controlled automatic emission-
testing apparatus.
We have demonstrated the advantages of using scandia-doped
tungsten powder to produce a matrix for the scandate cathode
instead of scandia-mixed matrix in our previous work [11–13]. In
order to further improve the emission properties of Sc-type
cathode, we prepared scandia-doped tungsten matrix with
spherical grain shape and a nano-sized dispersion of scandia in
the W matrix. The surface structure and emission property of this
kind of cathode have been studied.
3. Results and discussion
3.1. Characterization of scandia-doped tungsten matrix and cathode
2. Experimental
Fig. 1 shows the SEM images of powders of scandia-doped
tungsten (a) and pure tungsten (b). Pure tungsten powders have
polygon shape, whereas scandia-doped tungsten has spherical
shape. The particle size of scandia-doped tungsten is smaller than
that of pure tungsten. It is also found that part of nano-sized
scandia particles distribute among tungsten particles or exist on
the surface of tungsten particles. During the reduction process, the
Sc2O3-doped tungsten oxide powders are prepared by liqui-
d–liquid doping using a sol–gel technique. The content of Sc2O3 is
Ã
Corresponding author. Tel./fax: +8610 67391101.
E-mail address: wangjsh@bjut.edu.cn (J. Wang).
0022-3697/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2008.03.013

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Fig. 1. SEM images of (a) Sc2O3-doped W powders and (b) pure tungsten.
interface between tungsten produced by the reduction of tungsten
oxide and the unreacted tungsten oxide increases gradually. The
increase in the interface between the new crystal at the active
center and the tungsten oxide leads to the formation of defects in
tungsten oxide caused by the distortion, which is favorable for the
deposition of scandium oxide. The scandium oxide particles which
are deposited around tungsten grains hinder the growth of
tungsten grains during the reduction process by the pinning
effect. In addition, scandia covered at the surface of tungsten
particles hampers the deposition of gaseous tungsten oxide
hydrate on the tungsten particles or the chemical reaction at
some parts of the particles; as a result, the growth of tungsten
grains is retarded. Decrease in the particle size of the doped
tungsten powder is favorable not only for improving the
distribution homogeneity of scandium oxide in tungsten matrix,
but also for producing tungsten matrix with high porosity density
in which the active substance is easy to diffuse from the inner part
to the cathode surface.
The microstructures of the matrix and impregnated cathode
and related EDS results are displayed in Fig. 2. It can be seen that a
homogenous, porous matrix is formed with spherical grains with
a diameter of less than 1 mm and with many small particles
distributing uniformly around these big grains, see Fig. 2(a). Fig.
2(a) also shows that Sc2O3–W body has high porosity with a lot of
pores. These pores are formed during the sintering process. In the
initial period of sintering, new linking areas (sintering neck)
among particles are formed. These sintering necks grow with time
and block the grooves among the linking pores, so the individual
pores are formed. As for the impregnated cathode, a certain
amount of pores is indispensable in the matrix to meet the
requirement of Ba–Ca–aluminate impregnating. The EDS analysis
indicates that the big grains are tungsten, while the small round
particles in the size of a few nanometers are scandia (Fig. 2(b)),
indicating that sub-microstructure matrix with spherical grains
and homogenous distribution of scandia has been obtained. After
the matrix is impregnated with barium–calcium–aluminates at
high temperature and followed by ultrasonic water cleaning, the
morphology of the cathode surface changes and it is hard to find
dispersed Sc2O3 particles, see Fig. 2(c). We think that the
disappearance of Sc2O3 should result from a reaction between
Sc2O3 and Ba–Ca–aluminates which takes place during impreg-
nation, and the compounds formed are then dissolved and
removed by water cleaning.
was moved away from the cathode during activation to lower the
contamination by evaporants from the cathode and was normally
kept at a distance of 0.5 mm away from the cathode during
emission testing to minimize the effect of edge emission, which is
usual in cathode research.
Emission capability was tested with rectangular pulses with a
pulse width of 25 ms and 100 Hz. Cathode temperatures were
measured by a KELLER PV11 micro-optical pyrometer on the Mo
sleeve and are expressed as Mo-brightness temperatures in
this paper.
The cathode was activated at 1150 1Cb for about 3 h before
measurement. Typical I–V plots at temperatures from 700 to
900 1Cb are illustrated in Fig. 3. It can be seen from Fig. 3, that the
electron emission current density of direct deviation from the
space-charge limited region of Log I– Log U plot of this cathode can
reach 52 A/cm2 at 850 1Cb, higher than our previous report on the
emission current density of 34.3 A/cm2 at the same temperature
[11], and much higher than that of the ordinary scandate cathode
(the emission current density of direct deviation is 32 A/cm2 at
900 1Cb). It is found that the deviation starts at 487.6 A/cm2 at the
temperature of 900 1Cb. The Richardson work function F0 of our
cathode determined by low-temperature zero-field current den-
sities is around 1.1–1.2 eV, with Richardson constant A of 6–7 A/
cm2 K2. We have reported previously that the scandia-doped
matrix impregnated cathode has more than thousands of hours
lifetime at dc loading of 2 A/cm2 at 950 1Cb [12]. In order to find
out the emission stability of this kind of cathode in pulsed
condition, which is required by application in vacuum microwave
devices, a life test has been carried out with a continuously pulsed
loading of 70 A/cm2 at 950 1Cb, and the result is shown in Fig. 4. It
is encouraging to note that after the cathode has operated for
526 h under the above conditions, the emission property still
keeps almost stable, see Fig. 4. Thomas et al. reported that, as for
ordinary Ba–W cathode, it could operate for 50 h under the
condition of 45 A/cm2 at 1373 1C [14]. Therefore, Sc2O3–W matrix
impregnated cathode could provide a lifetime of about 10 times
longer with a continuously pulsed loading of current density
about 1/3 times higher at a temperature of about 300–400 1C
lower than Ba–W cathode.
3.3. Physical behavior of Sc
Fig. 5 shows Sc-concerned diffusing behaviors of Sc2O3-doped
W matrix before and after being impregnated with Ba, Ca and
aluminate. It can be seen that the content of Sc at the surface of
Sc2O3–W matrix keeps stable during the heat treatment period,
see Fig. 5a. Ion sputtering is added at this stage with an ion beam
of 3 kV, 0.6 mA at an area of 3 Â 3 mm2 and lasted for 3600 s. It is
found that oxygen exhibits a slight decrease whereas Sc increases
slightly after ion bombardment. It can be concluded from this
3.2. Emission properties
The emission measurements of the cathodes were done in
diode configuration. In the planar diode testing system, 3 mm
diameter cathodes were mounted into Mo sleeves and set in front
of a degassed Mo anode with an adjustable spacing. The anode

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Fig. 2. SEM image and EDS patterns for different samples. (a) Surface of Sc2O3-doped tungsten matrix, (b) EDS pattern of (a), (c) surface of impregnated cathode.
Temperature(°Cb)Slope
Deviation(A/cm2)
200
80
700
750
800
850
900
1.454
7.40
1.458
1.460
1.461
1.462
15.12
28.18
52.50
87.67*
25
4.5
1.5
0.5
0.1
10
20
60
100
200
Voltage (V)
500
3000
3000
3000
Fig. 3. Typical pulsed emission Log I– Log U plots for Sc2O3-doped W matrix dispenser cathode.
investigation that on the surface of Sc2O3-doped tungsten matrix
Sc2O3 keeps stable during heating up to 1150 1Cb or under ion
bombardment. As for the cathode which was prepared by
impregnating Sc2O3–W matrix with barium–calcium–aluminates
in a molar ratio of 4:1:1 (4BaO Á CaO ÁAl2O3) at temperatures of
1500–1700 1Cb, we are particularly interested to find by AES
analysis results that, in contrast to what has been found with
unimpregnated matrices, an obvious surface diffusion of Sc with
Ba and O is apparent at temperatures above 1000 1Cb during
heating, see Fig. 5b. That Sc in the matrix exists in the form of
Sc2O3 and the concent of Sc at matrix surface does not change
with the increase of temperature, see Fig. 5a, indicate that Sc

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could not diffuse in the form of Sc2O3, which is consistent with
Uda et. al.’s result. They claimed in their paper that Sc2O3 has very
low diffusion rate [15], so we believe that free or ionic Sc liberated
by chemical reactions during the period of impregnation and/or
activation diffuses from the inner part of the cathode to the
surface, leading to the increase of scandium at the cathode
surface.
For the ordinary impregnated cathodes, it has been widely
agreed that the lowest work function of the cathode will be
reached by an optimum coverage of Ba and O formed on tungsten
substrate in the form of a monolayer or partial monolayer [16,17].
For scandate cathode, Crombeen and Hasker [18] used the
monolayer model of a monolayer of Ba–Sc–O at scandate cathode
surface to explain its emission behavior, though it needs further
interpretation.
100
T=950°Cb, 5µs, 100Hz
80
In order to get the understanding of the surface layer of active
substance of Ba, Sc, O and the role of the surface layer in the
emission, depth profiles of Ba, Sc, O on W grains of a fully activated
cathode and the cathode, after current density degradated to 80% of
its original value by an accelerated life test under the condition of
pulsed loading of 100 A/cm2 at 1050 1Cb, were obtained using a PHI
700 Auger microprobe with an electron beam of 10 KV, and ion beam
of 500–1000 eV rastered over an area of 3 mm  3mm. The analysis
is carried out at room temperature after the cathode is transferred
from test system to analysis chamber.
60
40
20
0
0
100
200
300
400
500
600
Time of operation (h)
Fig. 6 illustrates the depth profiles taken from the activated
Sc2O3–W matrix cathode (a) and the cathode after accelerated life
Fig. 4. Life curve of 70 A/cm2 loading at 950 1Cb.
50
O
Sc
W
40
30
20
10
0
800
900
1000
1100
Heating Temp/°C
1150
Ion Bobm.
1150
1.2
O/W
Sc/W
Ba/W
0.8
0.4
0
25
800
1000
T/°Cb
1150, 15m
1150, 30m
Fig. 5. Comparison of Sc-concerned diffusing behaviors of Sc2O3-doped W matrix before and after being impregnated with Ba, Ca and aluminate. (a) Change of atomic
concentration at matrix surface during heating, (b) change of atomic concentration ratio at cathode surface during heating.

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100
80
100
80
60
40
20
0
Ba
Ba
Sc
O
Sc
O
W
W
60
40
20
0
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Sputter Depth (nm)
Sputter Depth (nm)
100
Ba
O
M-type cathode
Activated
80
60
40
20
0
W
Os
0
20
40
60
80
100
120
Sputter Depth (nm)
Fig. 6. Depth profiles of elements on the surface of Sc2O3–W fully activated cathode (a), depth profiles of elements on surface of the cathode after life test (b) and depth
profiles of elements on the surface of M-type cathode (c).
test (b) as well as from M-type cathode (c) for comparison. It is
clear that Ba, Sc and O cover the W substrate in thickness of about
80 nm, with Ba:Sc:O concentration ratios of about 1.9:1:2.2 at the
outermost surface, see Fig. 6a. After life test, the thickness of the
surface layer decreases to about 20 nm and the content of Sc
decreases with the change of the Ba:Sc:O concentration ratios at
the outermost surface to 2.6:1:4.2 (Fig. 6b). Analysis result
showed that the thickness of the monolayer of the M-type
cathode is about 10 nm (Fig. 6c), therefore, a Ba–Sc–O multilayer
having certain ratios of Ba:Sc:O is formed at the surface of
Sc2O3–W matrix cathode after activation, which is considered to
be responsible for copious emission capability. Both the decrease
of the thickness of active surface layer and the decrease of the
content of Sc at the surface could lead to the degradation of
current density.
We measured Auger peak-to-peak height ratios of Ba/W, O/W
and Sc/W at the surface of Sc2O3–W matrix impregnated cathode.
It shows that the APPH ratio of Ba/W for Sc2O3–W matrix
dispenser cathode is 1.35, almost four times higher than that of M-
type (S) cathodes (0.3, reported in literature [19]). Moreover, Sc/W
is 1.79, about two times higher than that of the monolayer of
oxidized Sc on W(10 0) (0.85, reported in literature [20]).
Combined with emission measurement result, we can conclude
that the cathode providing high emission property has high
content of active substances on the cathode surface.
is highly mobile are, therefore, the key to this kind of cathode, so
that scandium could efficiently diffuse to the cathode surface.
Spherical tungsten grains have low surface energy, so the growth
of tungsten grain could be retrained to a certain extent during
activation and operation periods and the sub-micrometer micro-
structure could be kept. Nanometer-sized Sc2O3 particles uni-
formly distributed in the matrix should be highly beneficial in
promoting the reactions that produce free Sc due to the large
interface area between the reaction compounds. Possible reac-
tions will be investigated in detail in our ongoing work.
4. Conclusions
(1) Scandia-doped porous tungsten impregnated cathode with a
homogeneous sub-micron matrix composing of W grains with
spherical shape and superfine Sc2O3 particles have been
obtained by liquid–liquid doping method.
(2) Scandia-doped tungsten sub-micron matrix dispenser cathode
has excellent emission property. Jdiv of 52 A/cm2 at 850 1Cb is
obtained.
(3) Sc diffuses from the inner part of the cathode to the surface in
the form of free scandium and forms a Ba–Sc–O multilayer
with Ba and O, which covers W framework after activation,
which leads to the copious emission property.
The above experimental results indicate that for the Sc2O3–W
matrix cathode, Sc at the surface comes from the inner part of the
cathode by diffusion, together with Ba and O, forming a surface
Ba–Sc–O multi-layer after activation. The thickness of the surface
layer and the content of Sc in this layer play an important role in
the emission. Chemical reactions that produce free or ionic Sc that
Acknowledgments
This work is sponsored by the National Nature Science
Foundation of China 50571001, China 863 program

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2006AA03Z524 and the Program for New Century Excellent
Talents in University.
[10] J. Gibson, G. Haas, R. Thomas, Investigation of scandate cathodes: emission,
fabrication and activation processes, IEEE Trans. Electron Devices 36 (1989)
209–214.
[11] W. Liu, K. Zhang, Y. Wang, K. Pan, X. Gu, J. Wang, J. Li, M. Zhou, Operating
model for scandia-doped matrix scandate cathode, Appl. Surf. Sci. 251 (2005)
80–88.
References
[12] H. Yuan, X. Gu, K. Pan, Y. Wang, W. Liu, K. Zhang, J. Wang, M. Zhou, J. Li,
Characteristics of scandate-impregnated cathodes with submicron Scandia-
doped matrices, Appl. Surf. Sci. 251 (2005) 106–113.
[13] J. Wang, L. Li, W. Liu, Y. Wang, L. Zhao, Y. Wang, Preparation and
characterization of scandia and Re-doped tungsten matrix impregnated
cathode, J. Phys. Chem. Solids 68 (2007) 2209–2215.
[14] R. Thomas, J. Gibson, G. Haas, R. Abrams, Thermionic sources for high-
brightness electron beams, IEEE Trans. Electron Devices 37 (1990) 850–861.
[15] E. Uda, O. Nakamura, S. Matsumoto, T. Higuchi, Emission and life
characteristics of thin film top-layer scandate cathode and diffusion of
Sc2O3 and W, Appl. Surf. Sci. 146 (1999) 31–38.
[16] A. Shih, G. Haas, C. Marrian, Preparation and oxidation of a thin Ba film, Appl.
Surf. Sci. 16 (1983) 93–105.
[17] K. Eyink, B. Lamartine, M. Lampert, T. Haas, Quantification of the surface
coverage of Ba and O on W substrates using Auger electron spectroscopy,
Appl. Surf. Sci. 20 (1985) 215–227.
[18] C. Crombeen, J. Hasker, Some experiments on the role of oxygen and surface
reactions for tungsten and scandate thermionic emitters, IEEE Trans. Electron
Devices 37 (1990) 2585–2588.
[19] A. Shroff, Review of dispenser cathodes, Rev. Tech. 23 (1991) 947–1026.
[20] T. Kawano, Y. Takai, R. Shimizu, Characterization of Sc–O/W (10 0) surface as
Schottky emitter: work function change for activation processing, Jpn. J. Appl.
Phys. 39 (2000) 577–580.
[1] A. Figner, A. Soloveichik, I. Judinskaya, Metal porous body having the pores
filled with Ba-Scandate, US Patent 3358178, filed 26.10.1964, granted
12.12.1967.
[2] J. Hasker, J. Van Esdonk, J.E. Crombeen, Properties and manufacture of top-
layer scandate cathodes, Appl. Surf. Sci. 26 (1986) 173–195.
[3] S. Yamamoto, I. Watanable, S. Taguchi, S. Sasaki, T. Yaguchi, Formation
mechanism of a monoatomic order surface layer on a Sc-type impregnated
cathode, IEEE Trans. Electron Devices 28 (1989) 490–494.
[4] U. van Slooten, P.A. Duine, Scanning Auger measurements of activated and
sputter cleaned Re-coated scandate cathodes, Appl. Surf. Sci. 111 (1997)
24–29.
[5] Y. Wang, T. Pan, F. Zhang, Preparation of thin film Sc-type cathode, Chinese
Patent CN 1128403A, 1996.
[6] G. Gartner, P. Geittner, H. Lydtin, A. Ritz, Emission properties of top-layer
scandate cathodes prepared by LAD, Appl. Surf. Sci. 111 (1997) 11–17.
[7] G. Gartner, P. Geittner, D. Raasch, Low temperature and cold emission of
scandate cathodes, Appl. Surf. Sci. 201 (2002) 61–68.
[8] S. Fukuda, K. Hayashi, S. Maeda, S. Michzono, Y. Saito, Performance of a high-
power klystron using a BI cathode in the KEK electron linac, Appl. Surf. Sci.
146 (1999) 84–88.
[9] J. Li, S. Yua, W. Shao, Q. Chen, M. Zhu, Investigation and application of
impregnated scandate cathodes, Appl. Surf. Sci. 215 (2003) 49–53.