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Journal of Alloys and Compounds 769 (2018) 706e712
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
High-quality LaxCeyPr1-x-yB6 single crystal with excellent thermionic
emission properties grown by optical floating zone melting method
Yan Wang, Jiuxing Zhang, Xinyu Yang*, Zhongwen Zhu, Jingjing Zhao, Bin Xu, Zhi Li
School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, Anhui, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 May 2018
Received in revised form
3 August 2018
Accepted 5 August 2018
Available online 6 August 2018
The high-quality LaxCeyPr1-x-yB6 (x ¼ 0.6e0.8, y ¼ 0.1e0.3) single crystals have been firstly grown by
optical floating zone melting method. The crystal structure, mechanical and thermionic emission
properties of the as-grown single crystals were characterized by X-ray single crystal diffractometer, SEM,
XRD, X-ray Laue diffraction. Subgrain-boundary-free [100] LaxCeyPr1-x-yB6 single crystal with a good
crystalline integrity and homogeneous element distribution is obtained. The Lattice constant of LaxCeyPr1-x-yB6 single crystal is lower than that of the LaB6 single crystal. The lowest FWHM of 162.0 arcsec
belongs to the La0.6Ce0.3Pr0.1B6 single crystal. The highest thermionic emission current density of 105.10
A/cm2 and the lowest work function of 2.562 eV indicate that La0.6Ce0.3Pr0.1B6 single crystal is a very
promising thermionic cathode material.
© 2018 Elsevier B.V. All rights reserved.
Keywords:
LaxCeyPr1-x-yB6 single crystal
Optical floating zone melting method
Thermionic emission properties
1. Introduction
Since its discovery in 1951 [1], lanthanum hexaboride (LaB6),
especially for LaB6 single crystal, has attracted significant attentions
because of meeting the almost desired properties required for
excellent cathode material, such as high brightness, low work
function, low volatility at high temperature and the possibility of
complete reactivation of emission property after poisoning, which
make the LaB6 single crystal cathode extensively used in electric
propulsion systems, scanning electron microscopes, university
research devices and so on [2e6]. With the development of the
deep-space exploration and high-power devices, the cathode material with high emission performance and long-life is necessary.
However, LaB6 single crystal cathode usually works at a high temperature of 1873 K, easily leading to the decrease in the service-life
of the cathode structure [7,8]. Therefore, how to further improve
the thermionic emission performance of material, making the
LaB6's emission properties at lower temperature can meet the
requirement of the devices, is an issue worth of research.
In recent decades, many attempts to improve the LaB6's emission properties by rare-earth elements doping [9e12], or addition
of the transition metal diboride MeB2 (Me ¼ Zr, V, Ti) [13e17], or
nanostructured LaB6 [18,19], have been carried out. Among these,
* Corresponding author.
E-mail address: xyyinuang@hfut.edu.cn (X. Yang).
https://doi.org/10.1016/j.jallcom.2018.08.046
0925-8388/© 2018 Elsevier B.V. All rights reserved.
doping the rare-earth elements with low work function to form the
multicomponent hexaboride single crystal has been proved to be
an effective method [11,12]. Up to now, several ternary hexaboride
single crystals, such as (LaxNd1-x)B6, (La0.54Ce0.46)B6, (La0.58Sm0.42)
B6, (LaxPr1-x)B6 [11,20], have been developed by the floating zone
melting or aluminum flux method. Among these, (La0.88Pr0.12)B6
prepared by aluminum flux method exhibits the highest current
density [20], which can be ascribed to the exclusion of subgrain
boundaries in (La0.88Pr0.12)B6 single crystal by doping of rare-earth
element Pr (the doping content of Pr < 0.3 at %) [10]. However,
aluminum flux method often introduces the Al impurity into the
hexaboride single crystals, weakening the purity and thermionic
emission properties of the single crystal [21]. Recently, Bao et al.
[22] reported that the high-purity (LaxCe1-x)B6 (x ¼ 0.2, 0.4, 0.6)
single crystals prepared by the floating zone melting method presented an excellent thermionic emission properties. Based on the
above, it can be concluded that the Ce or Pr dopants can effectively
improve the performance of LaB6 single crystal. However, there has
been no report about the preparation and performance of the (La,
Ce, Pr)B6 single crystal up to now, which is important to further
explore the thermionic emission potential of rare-earth hexaboride
and enhance the emission performance of LaB6 single crystal.
In this paper, the optical floating zone melting method (OFZM)
was employed to grow the high-quality LaxCeyPr1-x-yB6
(x ¼ 0.6e0.8, y ¼ 0.1e0.3) single crystals, and the characteristic,
mechanical and thermionic emission properties of single crystals
were systematically investigated.
Y. Wang et al. / Journal of Alloys and Compounds 769 (2018) 706e712
2. Experimental procedure
The raw materials were commercial LaB6 (99.9% purity), CeB6
(99.9% purity) and PrB6 (99.9% purity) powders. The powders were
weighted in accordance with the stoichiometry of LaxCeyPr1-x-yB6
(x ¼ 0.6e0.8, y ¼ 0.1e0.3). The refined mixed powders were obtained by high-energy ball milling for 3 h in Ar atmosphere. The
well-milled powders were then placed into a graphite die with
inner diameter of 20.4 mm to fabricate the feed rod using spark
plasma sintering system (LABOX-350, Sumitomo, Japan) at T ¼ 1973
K, P ¼ 40 MPa and t ¼ 5 min. The crystal growth was performed in
an optical floating zone furnace (FZ-T-2000-X-I-VPO-PC, Crystal
Systems Inc, Japan) equipped with four 5 KW Xenon lamps. The
[100] LaB6 single crystal was used as the seed rod, which was tied to
the lower shaft, and the feed rod was in the upper shaft. In order to
obtain the high-quality hexaboride single crystal, two-step floating
zone growth with the growth rate of 20 mm/h and 7 mm/h was
carried out, respectively. The images of the molten zone in the
growth process are shown in Fig. 1. Firstly, the feed rod and seed
crystal were placed in the focus center of the Xenon lamps and
when the output power of the Xenon lamp was increased to 10 KW,
the feed rod and seed crystal started to melt gradually, as shown in
Fig. 1(a). When the power was increased to 11.6 KW, the feed rod
melt was fallen down, connecting to the seed crystal and forming
the molten zone (Fig. 1(b)). Fig. 1(c) shows the stable growth of
single crystal, which is the precondition to obtain a high-quality
single crystal. In Fig. 1(d), the power was gradually decreased and
the width of the molten zone narrowed, and the crystal growth was
over until the molten zone was completely disconnected.
The single crystal X-ray diffraction (Gemini S Ultra, Oxford
diffraction, England) was utilized to analyze the crystal structure
and of the as-grown single crystals with enhanced Mo Ka radiation
(3.0 KW) at room temperature. The XRD (X Pert Pro MPD, PANalytical, Holland) was performed to execute phase analysis with Cu
Ka radiation using a step size of 0.02 and high-resolution X-ray
rocking curves of the as-grown LaxCeyPr1-x-yB6 single crystals were
recorded to check the crystal quality. The crystalline orientation
data were gathered by X-ray Laue diffraction (JF-3, Lidong, China).
The microstructure of the single crystal was analyzed by a scanning
electron microscope (Sigme, German Zeiss, Germany). The hardness of the single crystal was measured by a Vickers hardness tester
(DHV-1000Z, SCTMC, China).
The measurement of thermionic emission current density was
carried out by an electron emission measurement system (QX-350,
Qixing Vacuum, China). The distance between the single crystal
707
cathode and molybdenum anode is 1 mm. The single crystal cathode was treated with a carbon shield to ensure the electrons only
escaped from 1 mm2-area emitting surface. The emission current
densities were investigated at cathode temperatures of 1673, 1773
and 1873 K under the vacuum of 1 105 Pa and the cathode
temperature was measured using an optical pyrometer (WGG2201, SAIL, China).
3. Results and discussion
LaxCeyPr1-x-yB6 (x ¼ 0.6e0.8, y ¼ 0.1e0.3) quaternary hexaboride single crystals are grown by OFZM and the crystal images
are shown in Fig. 2. It can be seen that the as-grown single crystals
have uniform diameter, smooth surface and reddish purple color.
The size of these single crystals are 4.5e5 mm in diameter and
30e35 mm in length. The density of LaxCeyPr1-x-yB6 single crystal is
higher than that of LaB6 single crystal and the largest density of
4.758 g/cm3 belongs to the La0.6Ce0.1Pr0.3B6 single crystal, as listed
in Table 1.
Fig. 3(a)~(d) shows the typical single crystal X-ray diffraction
photographs of the as-grown LaxCeyPr1-x-yB6 crystals and it is
observed that all diffraction spots are independent, clear and no
splitting, indicating that the as-grown samples are typically single
crystals. Fig. 3(e)~(h) is corresponding lattice structure projection
maps along the <001> crystalline orientation of the LaxCeyPr1-x-yB6
single crystals. It can be seen that the LaxCeyPr1-x-yB6 single crystals
have a cubic structure. There are no miscellaneous points in the
Fig. 2. The images of the as-grown LaxCeyPr1-x-yB6 single crystals.
Fig. 1. The images of the molten zone established by OFZM: (a) rods started to melt, (b) the formation of molten zone, (c) the stable growth of single crystal, (d) the end of the
crystal growth.
708
Y. Wang et al. / Journal of Alloys and Compounds 769 (2018) 706e712
Table 1
Actual density, lattice constant and Vickers hardness of LaxCeyPr1-x-yB6 single crystals compared with LaB6 single crystal.
Sample
Actual density/(g/cm3)
Lattice constant/Å
Vickers hardness/GPa
LaB6
La0.8Ce0.1Pr0.1B6
La0.6Ce0.1Pr0.3B6
La0.6Ce0.2Pr0.2B6
La0.6Ce0.3Pr0.1B6
4.711
4.751
4.758
4.742
4.747
4.154
4.153
4.149
4.150
4.151
22.50
24.14
23.85
23.94
23.91
Fig. 3. Single crystal X-ray diffraction photographs of LaxCeyPr1-x-yB6 crystals: (a,e) La0.6Ce0.1Pr0.3B6, (b,f) La0.6Ce0.2Pr0.2B6, (c, g) La0.6Ce0.3Pr0.1B6, (d, h) La0.8Ce0.1Pr0.1B6.
projection map of lattice structure, which indicates that the highquality single crystals are obtained. The lattice constant of LaxCeyPr1-x-yB6 single crystal is lower than that of LaB6 single crystal,
which can be ascribed to the phenomenon of lanthanide contraction. Due to the r (La3þ) > r (Ce3þ) > r (Pr3þ), the lattice constants of
LaxCeyPr1-x-yB6 single crystals decrease with the increase in the
content of the doping element with lower ionic radius. Therefore,
the lattice constant of the La0.6Ce0.1Pr0.3B6 single crystal is the
smallest, as listed in Table 1.
The XRD patterns of single crystals with the transverse section
are shown in Fig. 4. It can be seen that characteristic diffraction
peaks of (100), (200), (300) for LaxCeyPr1-x-yB6 single crystals are
identified and no other phase are detected. In order to confirm
accurately the growth direction of the as-grown single crystals, the
X-ray Laue diffraction was performed to gather the crystalline
Fig. 4. XRD patterns of LaxCeyPr1-x-yB6 single crystals.
orientation data and the results of La0.6Ce0.3Pr0.1B6 single crystal, as
an example, are shown in Fig. 5. Fig. 5(a) shows the actual
diffraction pattern. The diffraction pattern presents a series of clear
and symmetrical spots rather than polycrystalline ring, further
indicating that the as-grown crystal is single crystal. The standard
diffraction spots (green dots) of (100) plane are shown in Fig. 5(b)
and it can be seen that the actual diffraction spots are perfectly
matched with the standard spots, indicating the growth direction
of single crystal is determined as <100> orientation, which agrees
well with the XRD results.
SEM image of La0.6Ce0.3Pr0.1B6 single crystal selected as representative is shown in Fig. 6(a). From the pattern, no obvious defects
and subgrain boundaries are observed, which indicates that highquality and subgrain-boundary-free hexaboride single crystal is
obtained by OFZM. The EDS mapping patterns, shown in Fig. 6(b)
~(e), indicate that the distribution of all elements is homogeneous.
High-resolution X-ray rocking curve is often used to evaluate the
crystal quality of single crystal [23]. Huet et al. [24] found that the
dislocation density of GaN crystal decreased with the decrease of
the full width at half-maximum (FWHM). Fig. 7 shows the rocking
curves of the as-grown LaxCeyPr1-x-yB6 single crystals with (100)
plane. All the characteristic diffraction peaks are symmetrical and
no split. It can be seen that La0.6Ce0.3Pr0.1B6 single crystal has the
lowest FWHM of 162.0 arcsec, indicating the best crystalline
integrity belongs to La0.6Ce0.3Pr0.1B6 single crystal.
The Vickers hardness of LaxCeyPr1-x-yB6 single crystals listed in
Table 1 indicates that the Vickers hardness of the quaternary hexaboride single crystals is higher than that of LaB6 single crystal,
which could be caused by solid solution strengthening of the
doping element.
Thermionic emission properties of the as-grown LaxCeyPr1-x-yB6
single crystals were studied based on the principle of the plate
diode. The thermionic emission current densities (J) of LaxCeyPr1-xyB6 single crystals with (100) plane were measured with the increase in the voltage (U) from 200 V to 4 kV at various cathode
temperatures and the J-U characteristic curves are plotted in Fig. 8.
Y. Wang et al. / Journal of Alloys and Compounds 769 (2018) 706e712
709
Fig. 5. X-ray Laue diffraction patterns of La0.6Ce0.3Pr0.1B6 single crystal: (a) the actual diffraction spots, (b) the matching result of the standard (green dots) and actual diffraction
spots. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. The EDS mapping patterns of La0.6Ce0.3Pr0.1B6 single crystal: (a) SEM image, (b) La, (c) Ce, (d) Pr, (e) B.
It can be seen that the emission current densities increase with the
increase in the cathode temperature or applied voltage. Fig. 8(a)
~(d) presents the J-U characteristic curves of La0.8Ce0.1Pr0.1B6,
La0.6Ce0.1Pr0.3B6, La0.6Ce0.2Pr0.2B6 and La0.6Ce0.3Pr0.1B6 single crystals, respectively, and the corresponding maximum emission current density is 89.01, 90.55, 79.92 and 105.10 A/cm2, which is
consistent with the crystal quality, as shown in Fig. 7. The same
preparation process is employed to fabricate the LaB6 single crystal
with (100) plane, the value of 65.1 A/cm2 at 1873 K is obviously
lower than that for LaxCeyPr1-x-yB6 single crystals, as shown in
Table 2. The highest current density for LaxCeyPr1-x-yB6 single
crystals is higher than that of 71.2 A/cm2 at 1873 K for La0.2Ce0.8B6
single crystal reported by Bao et al. [22], that of 56.81 A/cm2 at
1873 K for nanostructured LaB6 cathode reported by Zhou et al. [18].
The effective work function (Feff) of the as-grown LaxCeyPr1-xyB6 single crystals can be calculated by the Richardson-Dushman
formula as [25,26].
Therefore, Jo can be obtained by extrapolating the curves of lgJ
versus U0.5 to U0.5 ¼ 0, as shown in Fig. 9 and listed in Table 2. It can
be seen that the Jo of the as-grown LaxCeyPr1-x-yB6 single crystals is
higher than that of LaB6 and La0.2Ce0.8B6 single crystals. Based on
Eqs. (1) and (2), the values of Feff at different temperatures are
calculated and listed in Table 2. The La0.6Ce0.3Pr0.1B6 single crystal
has the lowest average effective work function of 2.562 eV, which is
smaller than that of 2.625 for LaB6 single crystal obtained at same
preparation process.
Therefore, the excellently thermionic property of LaxCeyPr1-xyB6 single crystals is mainly ascribed to the low work function.
Additionally, high quality of LaxCeyPr1-x-yB6 single crystals with the
subgrain-boundary-free due to the rare-earth element Pr dopant
and relatively low evaporation rate at a high temperature due to the
Ce dopant can also improve the thermionic property of LaxCeyPr1-xyB6 single crystals.
. feff ¼ KT ln 120:4T2 J0
4. Conclusions
(1)
where K is Boltzmann constant (K ¼ 8.617 105 eV/K), T is the
cathode temperature, Jo is the zero field emission current density
and can be obtained by the schottky effect as [27].
. lgJ ¼ lgJ0 þ 0:191 a0:5 T $U0:5
(2)
whereas a is the experimental constant related to the cathode
structure. From Eq. (2), it is generally considered that lgJ is linearly
dependent with U0.5 in the region of high applied voltage.
The high-quality LaxCeyPr1-x-yB6 (x ¼ 0.6e0.8, y ¼ 0.1e0.3)
quaternary hexaboride single crystals with excellent thermionic
emission properties have been grown by optical floating zone
melting method for the first time. The results of X-ray rocking curve
and SEM analysis indicate that subgrain-boundary-free LaxCeyPr1-xyB6 single crystals with a good crystalline integrity and homogeneous element distribution are obtained and the lowest FWHM of
162.0 arcsec belongs to the La0.6Ce0.3Pr0.1B6 single crystal. The XRD
and X-ray Laue diffraction data show that the LaxCeyPr1-x-yB6 single
crystals are all grown along the <100> orientation. The thermionic
emission results show that the maximum emission current density
710
Y. Wang et al. / Journal of Alloys and Compounds 769 (2018) 706e712
Fig. 7. X-ray rocking curves from (100) plane of LaxCeyPr1-x-yB6 single crystal: (a) La0.8Ce0.1Pr0.1B6, (b) La0.6Ce0.1Pr0.3B6, (c) La0.6Ce0.2Pr0.2B6 and (d) La0.6Ce0.3Pr0.1B6.
Fig. 8. Thermionic emission current densities of LaxCeyPr1-x-yB6 single crystals: (a) La0.8Ce0.1Pr0.1B6, (b) La0.6Ce0.1Pr0.3B6, (c) La0.6Ce0.2Pr0.2B6 and (d) La0.6Ce0.3Pr0.1B6.
Y. Wang et al. / Journal of Alloys and Compounds 769 (2018) 706e712
711
Table 2
Zero field emission current density Jo and effective work function Feff of the as-grown LaxCeyPr1-x-yB6, LaB6 and La0.2Ce0.8B6 single crystals at different temperatures.
T/K
La0.8Ce0.1Pr0.1B6a
Jo (A/cm2)
1673
6.006
1773
16.525
1873
38.650
Average Feff
a
b
La0.6Ce0.1Pr0.3B6a
La0.6Ce0.2Pr0.2B6a
La0.6Ce0.3Pr0.1B6a
LaB6a
La0.2Ce0.8B6b
Feff (eV)
Jo
Feff
Jo
Feff
Jo
Feff
Jo
Feff
Jo
Feff
2.572
2.590
2.616
2.593
6.909
14.554
39.402
2.552
2.609
2.613
2.591
5.667
16.800
35.636
2.581
2.587
2.629
2.599
9.093
19.496
40.480
2.513
2.564
2.608
2.562
4.878
14.187
28.989
2.602
2.612
2.662
2.625
4.890
12.300
36.300
2.602
2.634
2.626
2.621
This work.
Ref [28].
Fig. 9. Typical schottky plots (lgJ-U0.5 curves) of LaxCeyPr1-x-yB6 single crystals: (a) La0.8Ce0.1Pr0.1B6, (b) La0.6Ce0.1Pr0.3B6, (c) La0.6Ce0.2Pr0.2B6 and (d) La0.6Ce0.3Pr0.1B6.
of 105.10 A/cm2 at 1873 K and the lowest work function of 2.562 eV
belong to La0.6Ce0.3Pr0.1B6 single crystal.
Acknowledgements
Thanks are given to the National Natural Science Foundation of
China (51501051), Joint Foundation of Ministry of Education and
Equipment Pre-research of China (JD2017XAZC0002), Research
fund of the State Key Laboratory of Solidification Processing in
NWPU (SKLSP201510) and the Fundamental Research Funds for the
Central Universities (JZ2015HGBZ0091 and JD2016JGPY0005).
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