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A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser cathodes by using different precipitants
Accepted Manuscript A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser cathodes by using different precipitants Qiang Zheng, Yafen Shang, Yutao Cao, Jian Fang, Junshan Hu, Hao Fu, Dingming Yang, Yong Luo PII:
S0272-8842(19)30557-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.03.018
Reference:
CERI 20957
To appear in:
Ceramics International
Received Date: 4 December 2018 Revised Date:
15 February 2019
Accepted Date: 4 March 2019
Please cite this article as: Q. Zheng, Y. Shang, Y. Cao, J. Fang, J. Hu, H. Fu, D. Yang, Y. Luo, A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser cathodes by using different precipitants, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.03.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser
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cathodes by using different precipitants Qiang Zheng a, Yafen Shang a, Yutao Cao a, Jian Fang a, Junshan Hu a, Hao Fu a∗, Dingming Yang b∗, Yong Luo c
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a
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School of Physics, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China b School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621010, People’s Republic of China c School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China
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Abstract
In order to optimize the synthesis process of barium-calcium-aluminate (BCA) for the application of impregnated dispenser cathodes, the influence of two
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precipitants, ammonium carbonate (AC) and ammonium hydrogen carbonate (AHC),
AC C
on the chemical composition, phase transformation, thermal behaviors and morphology of the precursors was systematically investigated by means of XRD, TG-DSC, FESEM, and EDS mapping analysis. The results revealed that AC precipitant facilities to form NH4Al (OH)2CO3 (AACH) phase and barium calcium carbonate in the precursor, while the product precipitated by AHC comprises amorphous γ-AlOOH phase and the carbonate. Both precursors precipitated by the AC ∗
Author to whom correspondence should be addressed. Electronic mail:[email protected] and [email protected] 1
ACCEPTED MANUSCRIPT and AHC undergo various stages of phase transformations and finally converted to an emission-active material Ba3CaAl2O7 phase after sintering at 1200 ℃. DC emission current values have been measured and the maximum space charge limited current
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densities are 8.0 ± 0.3 and 6.5 ± 0.4 A/cm2 at 1130 ℃b for the testing cathodes impregnated with the AC and AHC precursor aluminates, respectively. The difference
in the emission capacity originates from the barium content in the aluminates prepared
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with different precipitants.
AC C
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capacity; impregnated dispenser cathode.
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Keywords: barium-calcium-aluminate; co-precipitate method; electron emission
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1. Introduction Impregnated dispenser cathodes (IDCs) are currently widely used in high-power vacuum electron devices (VEDs) due to their excellent thermionic emission capacity
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[1]. Generally, these cathodes provide current densities ranging from 1 to 15 A/cm2 depending upon the impregnant substance [2,3]. A basic dispenser cathode is
manufactured by capillary-filling impregnation of a porous tungsten matrix with the
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impregnant of molten barium-calcium-aluminate (BCA). The BCA continuously
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dispense barium and oxygen atoms onto the cathode surface to lower the surface work function and thus increase the thermionic emission.
Up to the present time, commercial BCA compositions include 5:3:2, 4:1:1, 6:1:2 and 3:1:1, where the notations signify the molar stoichiometric ratio coefficient of
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BaO: CaO: Al2O3. Among various commercial BCA compositions, the 6:1:2 type BCA is extensively used because of its reliability, reproducibility and high electronic emission [4,5]. Commercial 6:1:2 type BCA were commonly fabricated by a
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solid-state reaction with long heat treatment time at high temperature followed by
AC C
mechanical mixture [6]. Unfortunately, in such a process, the impregnants are difficult to be homogeneous due to the relatively large particle size and different densities of the raw materials of BaCO3, CaCO3 and Al2O3. Furthermore, impurities could be introduced into the impregnant from the grinding materials to result in the degradation of cathode performance. Liquid-chemical synthesis can mix individual components at the atomic level to reduce the diffusion distances and give rise to small grain size for the precursor
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ACCEPTED MANUSCRIPT powders, which facilitates to decrease sintering temperature and time [7,8,9]. Among all the wet-chemistry techniques, co-precipitation is considered as a maturing route for mass production of homogeneous and high purity BCA precursors with the
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advantages of simplicity, cost-effectiveness and inorganic reaction medium [10,11]. However, for the ternary BCA system, three metal cations, Ba2+, Ca2+, as well as Al3+, show different chemistries, which makes it difficult to obtain the expected chemical
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composition. For example, Li et al. synthesized 4:1:1 type BCA precursor by
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co-precipitation, but a large amount of calcium aluminate formed after calcining at 1200 ℃ due to the composition segregation [12]. Other researchers modified the preparation process but the issue was not solved thoroughly [13,14]. Up to now, a comprehensive understanding of co-precipitation process to adequately prepare
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high-grade BCA impregnants for high-end applications is still deficient. The process of co-precipitation reactions in other alumina containing systems, such as CeO2-ZrO2-Al2O3, and YAG, has been broadly investigated. The results
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demonstrated that the chemical composition and physical properties of precursor have
AC C
dramatic effects on the properties of the resultant oxide powders [15,16,17]. It was found that the properties of alumina powder prepared by precipitation dependon the chemical properties of precipitant severely [18,19]. However, to the best of our knowledge, the investigations on the effect of precipitants on the structure and properties of BCA were absent. The aim of the present work isto clarify the effect of different precipitants, ammonium carbonate (AC) and ammonium hydrogen carbonate (AHC), on the
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ACCEPTED MANUSCRIPT structure and propertiesof 6:1:2 type BCA powders synthesized by the co-precipitation. The influence of the precipitants on the chemical composition, phase transformation, thermal behavior, and morphology of the precursors are
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systematically investigated. Our results suggest that the IDCs impregnated with the 6:1:2 type BCA precipitated by AC preserve greater electronic emission capacity.
2.1
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2. Experimental Materials and synthesis procedure
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The starting materials, barium nitrate [Ba(NO3)2, purity＞99.9%], calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, purity＞99.9%], aluminum nitrate nonahydrate [Al(NO3)3·9H2O, purity＞99.9%], as well as the precipitants, ammonium carbonate
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(AC, purity＞99.0%) and ammonium hydrogen carbonate (AHC, purity＞99.0% ) were used for the co-precipitation method. All these chemicals were purchased from Chengdu Chron Chemicals Products (China) and were used as received without
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further purification.
AC C
AC (or AHC) solution with concentration of 2 mol/L was heated to 60 ℃ and poured into a mixture of Ba(NO3)2, Ca(NO3)2 and Al(NO3)3 with concentration of 0.2 mol/L at the same temperature as AC or AHC. The mole ratio of the Ba(NO3)2, Ca(NO3)2, and Al(NO3)3 is 6:1:4 in order to obtain the 6:1:2 type BCA. Magnetic stirring at 60 ℃ for 2 hours was utilized to ensure the completeness of the reactions and the homogeneity of the precipitates. During the process, the pH values of the final solutions were 8.6 and 7.4 when AC and AHC were used as the precipitants,
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ACCEPTED MANUSCRIPT respectively. Subsequently, the precipitate was aged for overnight at room temperature. Filtration was used to collect the precipitate from the solution. The collected precipitate was washed with de-ionized water at least five times and then dried at 80 ℃
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for 12 hours. The dried precursors were crushed into fine powders by agate mortar pestle. The powders were then pressed to cylindrical pellets with a diameter 15 mm. Finally, the
Characterization
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2.2
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pellets were calcined from 600 to 1200 ℃ for 3 hours in a muffle furnace.
Simultaneous thermo-gravimetric and differential scanning calorimeter (TG-DSC) analysis from room temperature to 1300 ℃ were performed on a NETZSCH, STA 449 F3 calorimeter with a scanning rate of 10 K/min under a nitrogen atmosphere in
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alumina crucibles. The phase composition present in the sample was examined by powder X-ray diffraction (XRD) using a diffractometer (DX-2700) with Cu Kα
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radiation in the step scanning mode between 2θ = 10 and 80 ° with a step size of 0.02 ° and collection time of 1s. The field scanning electron microscope (FESEM) images
AC C
and elemental distribution were characterized by using a Thermo Fisher, Inspect F50 FESEM equipped with an energy dispersive X-ray (EDX) analyzer. 2.3
Electronic emission capability measurements The details of the IDCs fabrication standard procedure have been reported
elsewhere [20]. The porosity of the tungsten (W) matrixes is controlled to be between 20 and 25 percent. The 6:1:2 type BCA is melted and impregnated into the W matrixes at 1650 ℃ for 3 minutes in a hydrogen atmosphere. The average 6
ACCEPTED MANUSCRIPT impregnation rate of the cathodes is 7.3% by weighing before and after impregnation. The diameter of the cathode pellets is 3 mm. They were heated by a tungsten heater
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coated with alumina.
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Figure 1. Schematic of the close-space diode (CSD) configuration for the emission measurement.
The electronic emission capacities of the IDCs were measured with a close-space diode (CSD) configuration in a dynamic ultrahigh vacuum system with pressure less
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than 5 × 10-6 Pa. The schematic diagram of the CSD with water-cooled anode is
AC C
presented in Fig. 1. The cathode temperature was detected by an optical pyrometer (KELLER HCW thermometer) and the emission current was measured through varying applied voltage U across the anode and cathode. The applied voltage (U) dependence the emission current density (J) in a space charge limited (SCL) CSD can be determined by the Child-Langmuir law (J = PU2/3) [21]. In most cases, IDCs operate in the SCL region which virtually eliminates the effects of variations in cathode temperature and work function. Therefore, the
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ACCEPTED MANUSCRIPT maximum SCL current density Jdiv, which was determined by noting the point of divergent from the linearity part of the log J–log U plots, is used for the evaluation of emission capability.
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3. Results and discussion
In the following sections, conventional cement chemistry abbreviations will be
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adopted for the various barium-calcium-aluminate (BCA) compounds, i. e. Al2O3,
BaO, and CaO will be indicated by the letters A, B and C, respectively. Formulas such
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as BaAl2O4, Ba3CaAl2O7 and substitutional solid solution of Ca in Ba4Al2O7 will appear as BA, B5CA and B4A (ss), respectively. 3.1
Thermal behavior of the precursors
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Simultaneous TG-DSC analyses were performed to study the mechanism of decompositions and phase transformation behaviors of the 6:1:2 BCA precursors precipitated by AC and AHC, respectively. Shown in Fig. 2 (a) and (b) are the heat
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flow and the normalized weight loss obtained with the heating rate of 10 K/min as a
AC C
function of temperature. In both graphs, the evolutions of the samples can be divided into three phases as indicated by the dash lines.
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Figure 2. (Color online) The TG/DSC traces of the 6:1:2 BCA precursors precipitated by AC (a) and AHC (b), respectively, with a heating rate of 10 K/min.
AC C
Four endothermic peaks occurring at 93.3, 218.7, 805.6 and 985.1 ℃ can be observed in Fig. 2 (a) for the precursor precipitated by AC. In phase A, the small peak at 93.3 ℃ can be attributed to the evaporation of absorptive water. There is an extra transition at 218.7 ℃ in Fig. 2 (a) compared with the DSC curve in Fig. 2 (b). It indicates that the two precipitants have different structures. XRD qualitative phase analysis [see Fig. 3 (a)] will verify that it is caused by the formation of NH4Al (OH)2CO3 (AACH) [22,23] in the precursor precipitated by AC. During 9
ACCEPTED MANUSCRIPT heating, the thermo-decomposition of AACH occurs and the reaction can be described by Equation (1). NH4Al (OH)2CO3→AlOOH+CO2↑+NH3↑+H2O↑
(1)
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Therefore, the weight loss of -11.5 % lower than 300 ℃ can be attributed to the evaporation of absorbed water and the decomposition of AACH. In phase B, two
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subtle endothermic peaks occurring at 805.6 and 985.1 ℃ in DSC curve can be assigned to the allotropic solid phase transformations of BaCO3 from the
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orthorhombic to the hexagonal and then to the cubic, respectively [24,25]. The weight of the sample decrease by -16.1 % until the temperature reaches 1100 ℃, which suggests that the decomposition of carbonate and the formation of aluminate carry on successively. In phase C, the DSC curve shows a series of small peaks close to each
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other, which may be caused by the baseline fluctuation of the DSC curve. The TG curve becomes smooth and suggests the completion of the decomposition of
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carbonate.
For the precursor precipitated by AHC [Fig. 2 (b)], only one endothermic peak
AC C
occurs at 93.3 ℃ in phase A', which corresponds to the evaporation of physically and/or chemically absorbed water. The weight loss by -10.2 % is a little lower than that for the precursor precipitated by AC in the same phase due to the absence of decomposition of AACH. In phase B', two endothermic peaks also refer to the allotropic solid phase transformations of BaCO3. These peak temperatures are very close to those in the DSC data of Fig. 2 (a), despite a small difference. It is worth noting that there is a small difference in weight loss (-16.9% vs -16.1 %) in the second 10
ACCEPTED MANUSCRIPT phase for the two samples. It can be interpreted by the difference in the solubility of calcium in the barium carbonate. As indicated by the following XRD analysis (see Fig. 3 and Table 2), the precursor precipitated by AC possesses larger cell parameters and
dioxide release compared with that precipitated by AHC. 3.2
Structural analysis
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more content in barium, which result in greater molecular weight and thus less carbon
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The powder X-ray diffraction (XRD) patterns of the co-precipitation precursors
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along with the samples calcined at different temperatures, 600, 800, 1000 and 1200℃, are shown in Fig. 3. The diffraction peaks of the component phases and their ICCD PDF reference numbers are also listed under the experimental patterns. The results are
AC C
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summarized in Table 1.
Figure 3. (Color online) The powder XRD patterns of the precursors precipitated by AC (a) and AHC (b) and the corresponding samples after calcination at various temperatures. The down arrow indicates the presence of AACH.
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ACCEPTED MANUSCRIPT Table 1 The component phases of the precursors precipitated by AC and AHC and the corresponding samples after calcination at various temperatures. Component Phases*
Samples
Precipitated by AHC
precursor calcinated at 600 ℃ calcinated at 800 ℃
AACH + (Ba,Ca)CO3 γ- AlOOH + (Ba,Ca)CO3 BA (ss) + (Ba,Ca)CO3
γ- AlOOH + (Ba,Ca)CO3 γ- AlOOH + (Ba,Ca)CO3 BA (ss) + (Ba,Ca)CO3
calcinated at 1000 ℃ calcinated at 1200 ℃
B3A (ss) + BA + (Ba,Ca)CO3 B3CA
B3A (ss) + BA + (Ba,Ca)CO3 B3CA
*
—γ- AlOOH is amorphous.
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Precipitated by AC
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Both the precursors precipitated by AC and AHC mainly exhibit broad peaks
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assigned to orthorhombic α-(Ba,Ca)CO3 phase (ICCD PDF# 05-0378). The
broadening of the peaks is caused by the small grain size due to the co-precipitation method [26,27]. Moreover, the XRD pattern for the precursor using AC also displays a broad peak at 2θ=15.2 ° as indicated by the down arrow in Fig. 3 (a), which
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corresponds to the peak of AACH (ICCD PDF# 29-0106). The low intensity of the diffraction peak can be ascribed to the small content and the poor crystallinity of
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AACH. As reported in Ref. [28], AACH can be obtained through the precipitation by the reaction between Al(NO3)3·9H2O and AC. The reaction can be described by
AC C
Equation (2).
Al(NO3)3+2(NH4)2CO3+H2O→NH4Al (OH)2CO3↓+3NH4NO3+CO2↑
(2)
However, for the precursor obtained by AHC, all the diffraction peaks can be attributed to just (Ba,Ca)CO3 phase and the peak at 2θ=15.2 ° attributed to AACH is absent. Therefore, we can deduce that Al3+cations exist in the amorphous form in the precursor. Unfortunately, scattering peaks responsible for the presence of amorphous phase cannot be observed. It is understandable due to the low content in the sample. 12
ACCEPTED MANUSCRIPT Du et. al [29] suggested that pH value has great effect on the reaction product of the aqueous Al(NO3)3 solution and the ammonium salt precipitant. Amorphous γ-AlOOH can be obtained in the pH value ranging from 6.5 to 7.6. However, when the
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pH value exceeds 8.5, AACH is the reaction product. In our experiments, the pH values of the reaction solutions were 7.4 and 8.6 when AHC and AC were used,
respectively. Therefore, the amorphous phase could be deduced as γ-AlOOH for the
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precursor precipitated by AHC.
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It can be seen from Fig. 3 (a) and (b) that the samples calcined at the same temperatures show nearly the same phase components. For example, when calcined at 600 ℃, both samples exhibit the characteristic peaks of orthorhombic α-(Ba,Ca)CO3 phase. Compared with the precursors, the diffraction peaks become sharper and some
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diffraction peaks, especially for the strongest peak near 2θ= 25 °, split into two or more peaks. These features illustrate that the crystallinity of (Ba,Ca)CO3 phase is improved by sintering. In addition, characteristic Bragg peak of AACH at 2θ = 15.2 °
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disappears for the AC precursors calcined at 600 ℃ and no extra diffraction peaks
AC C
except for those of (Ba,Ca)CO3 can be observed. It indicates that the AACH converts to the amorphous γ- AlOOH at this calcination temperature. After calcining at 800 ℃, BA (ss) (ICCD PDF# 17-0306) and BaCO3 phases can be observed. When temperature reaches 1000 ℃, B3A (ss) (ICCD PDF# 15-0074) presents along with BA (ss) and BaCO3 phases. These results consist with the previous work in the BaO-Al2O3 binary system [30,31]. The carbonate begins to react with the aluminum oxide when temperature is near 800 ℃ to generate BA (ss) phase. Then, BA (ss)
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ACCEPTED MANUSCRIPT phase reacts with the carbonate to produce the B3A (ss) phase. When temperature reaches 1000 ℃, the sintering product includes the B3A (ss), BaCO3 and small amount of residual BA (ss) phases. When the temperature increases to 1200 ℃, the
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B3A (ss) and BaCO3 phases react to form well-crystallized B3CA (ICCD PDF# 48-0569) phase.
From the TG-DSC and XRD studies the reaction sequence and phase
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as the following chemical equations and Table 1. (Ba,Ca)CO3+Al2O3→BA (ss)+CO2↑ BA (ss) can react with (Ba,Ca)CO3,
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transformations taking place for the formation of BCA can be reasonably summarized
(~800℃)
BA (ss)+(Ba,Ca)CO3→B3A (ss)+CO2↑(~1000℃)
(3)
(4)
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B3A (ss) and excessive BaCO3 phases react to form B3CA, B3A (ss)+(Ba,Ca)CO3→B3CA+CO2↑
(~1200℃)
(5)
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It is known that B3CA phase is the emission-active material due to its ability of being reduced by tungsten to release Ba and O atoms to reduce the work function of
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W matrix. B3CA derives from the B4A (ss) and can exist over a range of compositions as a result of Ba2+ replacement by Ca2+. Due to the greater ion radius of Ba2+ (1.35Å) than Ca2+ (1.00 Å), the B3CA compound containing more barium must have greater lattice parameters. The structural parameters including the lattice constants (a, b and c) and cell volume (V) of the two samples calcining at 1200 ℃ are calculated from the XRD patterns and the values are listed in Table 2. As shown in Table 2, the cell parameters of B3CA using AC precipitator are larger than that for 14
ACCEPTED MANUSCRIPT AHC precipitator. Consequently, the use of AC precipitator is beneficial to possess greater barium content in B3CA.
Precipitator
Jdiv (A) a
Cell Parameters
Phase a(Å)
b(Å)
c(Å)
V(Å3)
1130℃b
B3CA
11.6741
26.8070
11.1290
3493.59
8.0±0.3
AHC
B3CA
11.4800
26.4800
11.4800
3414.02
6.5±0.4
1050℃b
1000℃b
4.9±0.1
2.9±0.4
3.3±0.1
1.8±0.3
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AC
The standard deviations are calculated from five testing cathodes.
3.3
Morphological observation
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a—
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Table 2 The component phases, cell parameters, and the space charge-limited current density Jdiv of the 6:1:2 BCA using AC and AHC as precipitators.
The FESEM and EDS measurements were carried out to further investigate the effect of precipitants on the microstructure. Fig. 4 shows the FESEM images of the
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co-precipitation precursors obtained by using AC and AHC and the corresponding samples calcined at 1200 ℃, respectively. The EDS mapping of both precursors are
AC C
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shown in Fig. 5.
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Figure 4. The FESEM micrographs of the precursors (a and b) and calcined samples (c and
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d): (a) precipitated by AC; (b) precipitated by AHC; (c) 6:1:2 BCA precipitated by AC after
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calcined at 1200 ℃; (d) 6:1:2 BCA precipitated by AHC after calcined at 1200 ℃
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Figure 5. FESEM images of the precursors precipitated by AC (a) and AHC (e), and EDS mapping of Ba (b and f), Ca (c and g) and Al (d and h) elements.
AC C
It can be seen from Fig. 4 (a) and (b) that the precursor precipitated by AC consists of some particles surrounded by silk-like crystals. Whereas, the precursor precipitated by AHC comprise broccoli-like spherical particles and the matrix. The matrix is amorphous because not any specific configuration can be observed, which is in accordance with the XRD result [see Fig. 3 (b)]. As depicted in Fig. 5 (b)~(d), Al can be observed in the silk-like phase. Therefore, the silk-like phase can be ascribed to the AACH and the remaining particles are of (Ba,Ca)CO3. In Fig. 5 (f)~(h), the 16
ACCEPTED MANUSCRIPT broccoli-like spherical particles are abundant with Ba and Ca elements, however, the matrix is abundant with Al. Accordingly, the broccoli-like particles are of (Ba,Ca)CO3 and the matrix is of Al3+-contained salt.
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Micrograph of the precursors precipitated by AC and AHC calcined at 1200 ℃ are also studies by FESEM, and the results are shown in Fig. 4 (c) and (d),
respectively. It can be seen that the grain boundaries can be observed clearly for both
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samples. Additionally, the calcined aluminate precipitated by AC at 1200 ℃
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exhibited much lower agglomeration and rounder in shape than the sample precipitated by AHC and calcined at the same temperature. 3.4
Electronic emission capability of ICDs
The applied voltage (U) dependencies of emission current density (J) of the ICDs
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impregnated with 6:1:2 type BCA using AC and AHC as precipitators measured at 1000, 1050 and 1130 ℃b (℃b is brightness temperature) with DC mode are shown in Fig. 5 (a) and (b), respectively. The space charge-limited current density in term of
AC C
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Jdiv can be determined from Fig. 5 and the results are listed in Table 2.
Figure 5. (Color online) The log J-log U plots of the impregnated 6:1:2 type BCA ICDs with impregnants by using AC and AHC as precipitators. Measurements were performed at 1000, 1050, and 1130 ℃b with DC mode. 17
ACCEPTED MANUSCRIPT The Jdiv of the ICDs impregnated with 6:1:2 type BCA using AC precipitant reach 8.0±0.3 A/cm2 at 1130℃b. However, the Jdiv of the cathodes impregnated with aluminate using AHC is 6.5±0.4 A/cm2 at 1130 ℃b, which is about 23 % lower than
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that of the AC ICDs. It is well known that the emission capacity of ICDs depends on several factors, such as the porosity of W matrix, barium concentration and phase
compositions of BCA impregnant [1]. Generally, the Jdiv is mainly determined by the
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barium content of the BCA impregnant apart from the density of pores of the tungsten
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matrix [32]. In this work, the barium content of 6:1:2 type BCA precipitated by AC is higher than that prepared by AHC precipitant, which is confirmed from the difference in the cell parameters (see Table 2). Therefore, the main reason of the difference in the emission capacity of the two kinds of ICDs in this work can reasonably be
4. Conclusions
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attributed to the difference in the barium content.
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In summary, the effects of different precipitants of AC and AHC on the structure of precursors and the calcined aluminates, as well as the thermionic emission capacity
AC C
are systematically studied in this work. Our results indicate that the precursor precipitated by AC has the component phases of (Ba,Ca)CO3 particles and silk-like AACH crystals. In comparison, the precursor synthesized by AHC comprises broccoli-like (Ba,Ca)CO3 particles and the amorphous phase comprising Al3+. The two precursors exhibit similar thermal behaviors and phase evolution during calcining at different temperatures except for an extra peak attributed to the thermo-decomposition of AACH in the DSC curves. After calcining at 1200 ℃, both two aluminates 18
ACCEPTED MANUSCRIPT crystallized in the B3CA structure but have different barium content revealed from the cell parameters. DC current emission measurements present that the ICDs impregnated with the AC precursor aluminate possess current density of
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8.0 ± 0.3 A/cm2 at 1130 ℃b, which is greater than that of the ACH precursor BCA by about 23 %.The higher barium content in the former BCA is responsible for the
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superior emission capacity.
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Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51750110501).The authors thank Dr. Shengyi Yin from the Key Laboratory of High Power Microwave Sources and Technologies, Institute of Electronics, Chinese
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Academy of Sciences, for helpful discussions.
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[11] O. Wahyudi, F. Q. Zhang, F. Liu, et al., Morphological and phase analysis during the synthesis of Bi7Ti4NbO21 by a co-precipitation method, Ceram. Int. 41 (2015) S41-S46. [12] N. Li, N. C. Luhmann Jr. , D. Gamzina, et al., Study on the compositions of emission active materials and the emission properties of dispenser cathodes made from Scandia-doped powders,
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[13] J. X. Bao, B. F. Wang, P. J. Wang, Preparation and performance analysis of barium dispenser cathodes, Vacuum 81 (2007) 1029-1034.
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[14] C. Y. Shen, Z.S. Li, T. Qiu, et al., High property aluminate for Ba-W cathode by impregnation,
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Rare. Metal Mat. Eng. 36 ( 2007) 2057-2060 (in Chinese). [15] X. Han, Z. Liang, L. Feng, et al., Co-precipitated synthesis of Al2O3-ZrO2 composite ceramic nanopowders by precipitant and drying method regulation: A systematic study, Ceram. Int. 41 (2015) 505-513.
[16] H. Wang, C. Huang, Q. Deng, et al., Effect of co-precipitation method on microwave dielectric properties of 0.95(Ca0.88Sr0.12)TiO3-0.05(Bi0.5Na0.5)TiO3 ceramics, Ceram. Int.44 (2018) 8700-8705. [17] C. Marlot, E. Barraud, S. L. Gallet, et al., Synthesis of YAG nanopowder by the 20
ACCEPTED MANUSCRIPT co-precipitation method: Influence of pH and study of the reaction mechanisms, J. Solid State Chem. 191 (2012) 114-120. [18] D. C. Shin, S. S. Park, J. H. Kim, et al., Study on α-alumina precursors prepared using different ammonium salt precipitants, J. Ind. Eng. Chem. 20 (2014) 1269-1275.
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[19] S. Ghanizadeh, X. Bao, B. Vaidhyanathan, et al., Synthesis of nano α-alumina powders using hydrothermal and precipitation routes: a comparative study, Ceram. Int. 40 (2014) 1311-1319.
[20] S. Taguchi, T. Aida, S. Yamamoto, Investigation of Sc2O3 mixed-matrix Ba-Ca-Aluminate
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impregnated cathodes, IEEE Trans. Electron Devices ED-31 (1984) 900-903.
[21] A. S. Gilmour, Jr. , Klystrons, traveling wave tubes, magnetrons, crossed-field amplifiers, and gyrotrons, Artech House, Boston/ London, 2011.
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[22] P. Su, X. Guo, S. Ji, Effect of multi component catalyzer on preparation of ultrafine α-alumina at low sintering temperature, Adv. Powder Technol. 20 (2009) 542-547. [23 ]M.Rahmani, O. Mirzaee, M.Tajally, The effects of pH and excess Al3+ content on the microstructure and phase volution of YAG polycrystals, Ceram. Int. 43 (2017) 12563-12571.
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[24] R.B.Fahim, M.I.Zaki, G.A.M.Hussien, Effect of processing parameters on the kinetics of decomposition of certain simple anhydrous carbonates, Powder Technol. 33 (1982) 161-165. [25] M. I. Zaki, G. A. M. Hussien, R. B. Fahim, Characterization of the powder mixture of the reaction between alumina and barium carbonate, J. Mater. Sci. Lett. 4 (1985) 517-522.
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[26]M. S. Toprak, M.Darab,G. E.Syvertsen, et al., Synthesis of nanostructured BSCF by oxalate co-precipitation-As potential cathode material for solid oxide fuels cells, Int. J. Hydrogen Energ.
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35 (2010) 9448-9454.
[27] M. Asgari, A. Ataie, H. Sabarou, Synthesis of BaCO3nano-particles through a polymer-assisted precipitation method, J. Mater. Eng. Perform. 23 (2014) 826-832. [28] S. Li, J. Deng, L. Xiong, et al., Design and construct CeO2-ZrO2-Al2O3 materials with controlled structures via co-precipitation method by using different precipitants, Ceram. Int.44 (2018) 20929-20938. [29] X. Du, Y. Wang, X. Su, et al., Influences of pH value on the microstructure and phase transformation of aluminum hydroxide, Powder Technol. 192 (2009) 40-46. 21
ACCEPTED MANUSCRIPT [30] J. Beretka, T. Brown, Studies of the intermediate phases in the reaction between barium carbonate and aluminium oxide, Aust. J. Chem. 26 (1973) 2527-2531. [31] M. Mohapatra, D. M. Pattanaik, S. Anand, et al., Effect of barium to aluminium ratio on phases leading to barium aluminates, Ceram. Int.33 (2007) 531-535.
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[32] J. L. Cronin, Modern dispenser cathode, IEE Proc. 128 (1981) 19-32.
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S0272-8842(19)30557-7
DOI:
https://doi.org/10.1016/j.ceramint.2019.03.018
Reference:
CERI 20957
To appear in:
Ceramics International
Received Date: 4 December 2018 Revised Date:
15 February 2019
Accepted Date: 4 March 2019
Please cite this article as: Q. Zheng, Y. Shang, Y. Cao, J. Fang, J. Hu, H. Fu, D. Yang, Y. Luo, A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser cathodes by using different precipitants, Ceramics International (2019), doi: https:// doi.org/10.1016/j.ceramint.2019.03.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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A comparative study of the co-precipitate synthesis of barium-calcium-aluminates impregnants for dispenser
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cathodes by using different precipitants Qiang Zheng a, Yafen Shang a, Yutao Cao a, Jian Fang a, Junshan Hu a, Hao Fu a∗, Dingming Yang b∗, Yong Luo c
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a
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School of Physics, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China b School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, 621010, People’s Republic of China c School of Electronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu, 610054, People’s Republic of China
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Abstract
In order to optimize the synthesis process of barium-calcium-aluminate (BCA) for the application of impregnated dispenser cathodes, the influence of two
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precipitants, ammonium carbonate (AC) and ammonium hydrogen carbonate (AHC),
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on the chemical composition, phase transformation, thermal behaviors and morphology of the precursors was systematically investigated by means of XRD, TG-DSC, FESEM, and EDS mapping analysis. The results revealed that AC precipitant facilities to form NH4Al (OH)2CO3 (AACH) phase and barium calcium carbonate in the precursor, while the product precipitated by AHC comprises amorphous γ-AlOOH phase and the carbonate. Both precursors precipitated by the AC ∗
Author to whom correspondence should be addressed. Electronic mail:[email protected] and [email protected] 1
ACCEPTED MANUSCRIPT and AHC undergo various stages of phase transformations and finally converted to an emission-active material Ba3CaAl2O7 phase after sintering at 1200 ℃. DC emission current values have been measured and the maximum space charge limited current
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densities are 8.0 ± 0.3 and 6.5 ± 0.4 A/cm2 at 1130 ℃b for the testing cathodes impregnated with the AC and AHC precursor aluminates, respectively. The difference
in the emission capacity originates from the barium content in the aluminates prepared
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with different precipitants.
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capacity; impregnated dispenser cathode.
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Keywords: barium-calcium-aluminate; co-precipitate method; electron emission
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1. Introduction Impregnated dispenser cathodes (IDCs) are currently widely used in high-power vacuum electron devices (VEDs) due to their excellent thermionic emission capacity
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[1]. Generally, these cathodes provide current densities ranging from 1 to 15 A/cm2 depending upon the impregnant substance [2,3]. A basic dispenser cathode is
manufactured by capillary-filling impregnation of a porous tungsten matrix with the
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impregnant of molten barium-calcium-aluminate (BCA). The BCA continuously
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dispense barium and oxygen atoms onto the cathode surface to lower the surface work function and thus increase the thermionic emission.
Up to the present time, commercial BCA compositions include 5:3:2, 4:1:1, 6:1:2 and 3:1:1, where the notations signify the molar stoichiometric ratio coefficient of
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BaO: CaO: Al2O3. Among various commercial BCA compositions, the 6:1:2 type BCA is extensively used because of its reliability, reproducibility and high electronic emission [4,5]. Commercial 6:1:2 type BCA were commonly fabricated by a
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solid-state reaction with long heat treatment time at high temperature followed by
AC C
mechanical mixture [6]. Unfortunately, in such a process, the impregnants are difficult to be homogeneous due to the relatively large particle size and different densities of the raw materials of BaCO3, CaCO3 and Al2O3. Furthermore, impurities could be introduced into the impregnant from the grinding materials to result in the degradation of cathode performance. Liquid-chemical synthesis can mix individual components at the atomic level to reduce the diffusion distances and give rise to small grain size for the precursor
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ACCEPTED MANUSCRIPT powders, which facilitates to decrease sintering temperature and time [7,8,9]. Among all the wet-chemistry techniques, co-precipitation is considered as a maturing route for mass production of homogeneous and high purity BCA precursors with the
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advantages of simplicity, cost-effectiveness and inorganic reaction medium [10,11]. However, for the ternary BCA system, three metal cations, Ba2+, Ca2+, as well as Al3+, show different chemistries, which makes it difficult to obtain the expected chemical
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composition. For example, Li et al. synthesized 4:1:1 type BCA precursor by
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co-precipitation, but a large amount of calcium aluminate formed after calcining at 1200 ℃ due to the composition segregation [12]. Other researchers modified the preparation process but the issue was not solved thoroughly [13,14]. Up to now, a comprehensive understanding of co-precipitation process to adequately prepare
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high-grade BCA impregnants for high-end applications is still deficient. The process of co-precipitation reactions in other alumina containing systems, such as CeO2-ZrO2-Al2O3, and YAG, has been broadly investigated. The results
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demonstrated that the chemical composition and physical properties of precursor have
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dramatic effects on the properties of the resultant oxide powders [15,16,17]. It was found that the properties of alumina powder prepared by precipitation dependon the chemical properties of precipitant severely [18,19]. However, to the best of our knowledge, the investigations on the effect of precipitants on the structure and properties of BCA were absent. The aim of the present work isto clarify the effect of different precipitants, ammonium carbonate (AC) and ammonium hydrogen carbonate (AHC), on the
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ACCEPTED MANUSCRIPT structure and propertiesof 6:1:2 type BCA powders synthesized by the co-precipitation. The influence of the precipitants on the chemical composition, phase transformation, thermal behavior, and morphology of the precursors are
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systematically investigated. Our results suggest that the IDCs impregnated with the 6:1:2 type BCA precipitated by AC preserve greater electronic emission capacity.
2.1
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2. Experimental Materials and synthesis procedure
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The starting materials, barium nitrate [Ba(NO3)2, purity＞99.9%], calcium nitrate tetrahydrate [Ca(NO3)2·4H2O, purity＞99.9%], aluminum nitrate nonahydrate [Al(NO3)3·9H2O, purity＞99.9%], as well as the precipitants, ammonium carbonate
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(AC, purity＞99.0%) and ammonium hydrogen carbonate (AHC, purity＞99.0% ) were used for the co-precipitation method. All these chemicals were purchased from Chengdu Chron Chemicals Products (China) and were used as received without
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further purification.
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AC (or AHC) solution with concentration of 2 mol/L was heated to 60 ℃ and poured into a mixture of Ba(NO3)2, Ca(NO3)2 and Al(NO3)3 with concentration of 0.2 mol/L at the same temperature as AC or AHC. The mole ratio of the Ba(NO3)2, Ca(NO3)2, and Al(NO3)3 is 6:1:4 in order to obtain the 6:1:2 type BCA. Magnetic stirring at 60 ℃ for 2 hours was utilized to ensure the completeness of the reactions and the homogeneity of the precipitates. During the process, the pH values of the final solutions were 8.6 and 7.4 when AC and AHC were used as the precipitants,
5
ACCEPTED MANUSCRIPT respectively. Subsequently, the precipitate was aged for overnight at room temperature. Filtration was used to collect the precipitate from the solution. The collected precipitate was washed with de-ionized water at least five times and then dried at 80 ℃
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for 12 hours. The dried precursors were crushed into fine powders by agate mortar pestle. The powders were then pressed to cylindrical pellets with a diameter 15 mm. Finally, the
Characterization
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2.2
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pellets were calcined from 600 to 1200 ℃ for 3 hours in a muffle furnace.
Simultaneous thermo-gravimetric and differential scanning calorimeter (TG-DSC) analysis from room temperature to 1300 ℃ were performed on a NETZSCH, STA 449 F3 calorimeter with a scanning rate of 10 K/min under a nitrogen atmosphere in
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alumina crucibles. The phase composition present in the sample was examined by powder X-ray diffraction (XRD) using a diffractometer (DX-2700) with Cu Kα
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radiation in the step scanning mode between 2θ = 10 and 80 ° with a step size of 0.02 ° and collection time of 1s. The field scanning electron microscope (FESEM) images
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and elemental distribution were characterized by using a Thermo Fisher, Inspect F50 FESEM equipped with an energy dispersive X-ray (EDX) analyzer. 2.3
Electronic emission capability measurements The details of the IDCs fabrication standard procedure have been reported
elsewhere [20]. The porosity of the tungsten (W) matrixes is controlled to be between 20 and 25 percent. The 6:1:2 type BCA is melted and impregnated into the W matrixes at 1650 ℃ for 3 minutes in a hydrogen atmosphere. The average 6
ACCEPTED MANUSCRIPT impregnation rate of the cathodes is 7.3% by weighing before and after impregnation. The diameter of the cathode pellets is 3 mm. They were heated by a tungsten heater
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coated with alumina.
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Figure 1. Schematic of the close-space diode (CSD) configuration for the emission measurement.
The electronic emission capacities of the IDCs were measured with a close-space diode (CSD) configuration in a dynamic ultrahigh vacuum system with pressure less
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than 5 × 10-6 Pa. The schematic diagram of the CSD with water-cooled anode is
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presented in Fig. 1. The cathode temperature was detected by an optical pyrometer (KELLER HCW thermometer) and the emission current was measured through varying applied voltage U across the anode and cathode. The applied voltage (U) dependence the emission current density (J) in a space charge limited (SCL) CSD can be determined by the Child-Langmuir law (J = PU2/3) [21]. In most cases, IDCs operate in the SCL region which virtually eliminates the effects of variations in cathode temperature and work function. Therefore, the
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ACCEPTED MANUSCRIPT maximum SCL current density Jdiv, which was determined by noting the point of divergent from the linearity part of the log J–log U plots, is used for the evaluation of emission capability.
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3. Results and discussion
In the following sections, conventional cement chemistry abbreviations will be
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adopted for the various barium-calcium-aluminate (BCA) compounds, i. e. Al2O3,
BaO, and CaO will be indicated by the letters A, B and C, respectively. Formulas such
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as BaAl2O4, Ba3CaAl2O7 and substitutional solid solution of Ca in Ba4Al2O7 will appear as BA, B5CA and B4A (ss), respectively. 3.1
Thermal behavior of the precursors
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Simultaneous TG-DSC analyses were performed to study the mechanism of decompositions and phase transformation behaviors of the 6:1:2 BCA precursors precipitated by AC and AHC, respectively. Shown in Fig. 2 (a) and (b) are the heat
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flow and the normalized weight loss obtained with the heating rate of 10 K/min as a
AC C
function of temperature. In both graphs, the evolutions of the samples can be divided into three phases as indicated by the dash lines.
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Figure 2. (Color online) The TG/DSC traces of the 6:1:2 BCA precursors precipitated by AC (a) and AHC (b), respectively, with a heating rate of 10 K/min.
AC C
Four endothermic peaks occurring at 93.3, 218.7, 805.6 and 985.1 ℃ can be observed in Fig. 2 (a) for the precursor precipitated by AC. In phase A, the small peak at 93.3 ℃ can be attributed to the evaporation of absorptive water. There is an extra transition at 218.7 ℃ in Fig. 2 (a) compared with the DSC curve in Fig. 2 (b). It indicates that the two precipitants have different structures. XRD qualitative phase analysis [see Fig. 3 (a)] will verify that it is caused by the formation of NH4Al (OH)2CO3 (AACH) [22,23] in the precursor precipitated by AC. During 9
ACCEPTED MANUSCRIPT heating, the thermo-decomposition of AACH occurs and the reaction can be described by Equation (1). NH4Al (OH)2CO3→AlOOH+CO2↑+NH3↑+H2O↑
(1)
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Therefore, the weight loss of -11.5 % lower than 300 ℃ can be attributed to the evaporation of absorbed water and the decomposition of AACH. In phase B, two
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subtle endothermic peaks occurring at 805.6 and 985.1 ℃ in DSC curve can be assigned to the allotropic solid phase transformations of BaCO3 from the
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orthorhombic to the hexagonal and then to the cubic, respectively [24,25]. The weight of the sample decrease by -16.1 % until the temperature reaches 1100 ℃, which suggests that the decomposition of carbonate and the formation of aluminate carry on successively. In phase C, the DSC curve shows a series of small peaks close to each
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other, which may be caused by the baseline fluctuation of the DSC curve. The TG curve becomes smooth and suggests the completion of the decomposition of
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carbonate.
For the precursor precipitated by AHC [Fig. 2 (b)], only one endothermic peak
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occurs at 93.3 ℃ in phase A', which corresponds to the evaporation of physically and/or chemically absorbed water. The weight loss by -10.2 % is a little lower than that for the precursor precipitated by AC in the same phase due to the absence of decomposition of AACH. In phase B', two endothermic peaks also refer to the allotropic solid phase transformations of BaCO3. These peak temperatures are very close to those in the DSC data of Fig. 2 (a), despite a small difference. It is worth noting that there is a small difference in weight loss (-16.9% vs -16.1 %) in the second 10
ACCEPTED MANUSCRIPT phase for the two samples. It can be interpreted by the difference in the solubility of calcium in the barium carbonate. As indicated by the following XRD analysis (see Fig. 3 and Table 2), the precursor precipitated by AC possesses larger cell parameters and
dioxide release compared with that precipitated by AHC. 3.2
Structural analysis
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more content in barium, which result in greater molecular weight and thus less carbon
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The powder X-ray diffraction (XRD) patterns of the co-precipitation precursors
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along with the samples calcined at different temperatures, 600, 800, 1000 and 1200℃, are shown in Fig. 3. The diffraction peaks of the component phases and their ICCD PDF reference numbers are also listed under the experimental patterns. The results are
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summarized in Table 1.
Figure 3. (Color online) The powder XRD patterns of the precursors precipitated by AC (a) and AHC (b) and the corresponding samples after calcination at various temperatures. The down arrow indicates the presence of AACH.
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ACCEPTED MANUSCRIPT Table 1 The component phases of the precursors precipitated by AC and AHC and the corresponding samples after calcination at various temperatures. Component Phases*
Samples
Precipitated by AHC
precursor calcinated at 600 ℃ calcinated at 800 ℃
AACH + (Ba,Ca)CO3 γ- AlOOH + (Ba,Ca)CO3 BA (ss) + (Ba,Ca)CO3
γ- AlOOH + (Ba,Ca)CO3 γ- AlOOH + (Ba,Ca)CO3 BA (ss) + (Ba,Ca)CO3
calcinated at 1000 ℃ calcinated at 1200 ℃
B3A (ss) + BA + (Ba,Ca)CO3 B3CA
B3A (ss) + BA + (Ba,Ca)CO3 B3CA
*
—γ- AlOOH is amorphous.
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Precipitated by AC
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Both the precursors precipitated by AC and AHC mainly exhibit broad peaks
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assigned to orthorhombic α-(Ba,Ca)CO3 phase (ICCD PDF# 05-0378). The
broadening of the peaks is caused by the small grain size due to the co-precipitation method [26,27]. Moreover, the XRD pattern for the precursor using AC also displays a broad peak at 2θ=15.2 ° as indicated by the down arrow in Fig. 3 (a), which
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corresponds to the peak of AACH (ICCD PDF# 29-0106). The low intensity of the diffraction peak can be ascribed to the small content and the poor crystallinity of
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AACH. As reported in Ref. [28], AACH can be obtained through the precipitation by the reaction between Al(NO3)3·9H2O and AC. The reaction can be described by
AC C
Equation (2).
Al(NO3)3+2(NH4)2CO3+H2O→NH4Al (OH)2CO3↓+3NH4NO3+CO2↑
(2)
However, for the precursor obtained by AHC, all the diffraction peaks can be attributed to just (Ba,Ca)CO3 phase and the peak at 2θ=15.2 ° attributed to AACH is absent. Therefore, we can deduce that Al3+cations exist in the amorphous form in the precursor. Unfortunately, scattering peaks responsible for the presence of amorphous phase cannot be observed. It is understandable due to the low content in the sample. 12
ACCEPTED MANUSCRIPT Du et. al [29] suggested that pH value has great effect on the reaction product of the aqueous Al(NO3)3 solution and the ammonium salt precipitant. Amorphous γ-AlOOH can be obtained in the pH value ranging from 6.5 to 7.6. However, when the
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pH value exceeds 8.5, AACH is the reaction product. In our experiments, the pH values of the reaction solutions were 7.4 and 8.6 when AHC and AC were used,
respectively. Therefore, the amorphous phase could be deduced as γ-AlOOH for the
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precursor precipitated by AHC.
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It can be seen from Fig. 3 (a) and (b) that the samples calcined at the same temperatures show nearly the same phase components. For example, when calcined at 600 ℃, both samples exhibit the characteristic peaks of orthorhombic α-(Ba,Ca)CO3 phase. Compared with the precursors, the diffraction peaks become sharper and some
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diffraction peaks, especially for the strongest peak near 2θ= 25 °, split into two or more peaks. These features illustrate that the crystallinity of (Ba,Ca)CO3 phase is improved by sintering. In addition, characteristic Bragg peak of AACH at 2θ = 15.2 °
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disappears for the AC precursors calcined at 600 ℃ and no extra diffraction peaks
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except for those of (Ba,Ca)CO3 can be observed. It indicates that the AACH converts to the amorphous γ- AlOOH at this calcination temperature. After calcining at 800 ℃, BA (ss) (ICCD PDF# 17-0306) and BaCO3 phases can be observed. When temperature reaches 1000 ℃, B3A (ss) (ICCD PDF# 15-0074) presents along with BA (ss) and BaCO3 phases. These results consist with the previous work in the BaO-Al2O3 binary system [30,31]. The carbonate begins to react with the aluminum oxide when temperature is near 800 ℃ to generate BA (ss) phase. Then, BA (ss)
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ACCEPTED MANUSCRIPT phase reacts with the carbonate to produce the B3A (ss) phase. When temperature reaches 1000 ℃, the sintering product includes the B3A (ss), BaCO3 and small amount of residual BA (ss) phases. When the temperature increases to 1200 ℃, the
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B3A (ss) and BaCO3 phases react to form well-crystallized B3CA (ICCD PDF# 48-0569) phase.
From the TG-DSC and XRD studies the reaction sequence and phase
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as the following chemical equations and Table 1. (Ba,Ca)CO3+Al2O3→BA (ss)+CO2↑ BA (ss) can react with (Ba,Ca)CO3,
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transformations taking place for the formation of BCA can be reasonably summarized
(~800℃)
BA (ss)+(Ba,Ca)CO3→B3A (ss)+CO2↑(~1000℃)
(3)
(4)
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B3A (ss) and excessive BaCO3 phases react to form B3CA, B3A (ss)+(Ba,Ca)CO3→B3CA+CO2↑
(~1200℃)
(5)
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It is known that B3CA phase is the emission-active material due to its ability of being reduced by tungsten to release Ba and O atoms to reduce the work function of
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W matrix. B3CA derives from the B4A (ss) and can exist over a range of compositions as a result of Ba2+ replacement by Ca2+. Due to the greater ion radius of Ba2+ (1.35Å) than Ca2+ (1.00 Å), the B3CA compound containing more barium must have greater lattice parameters. The structural parameters including the lattice constants (a, b and c) and cell volume (V) of the two samples calcining at 1200 ℃ are calculated from the XRD patterns and the values are listed in Table 2. As shown in Table 2, the cell parameters of B3CA using AC precipitator are larger than that for 14
ACCEPTED MANUSCRIPT AHC precipitator. Consequently, the use of AC precipitator is beneficial to possess greater barium content in B3CA.
Precipitator
Jdiv (A) a
Cell Parameters
Phase a(Å)
b(Å)
c(Å)
V(Å3)
1130℃b
B3CA
11.6741
26.8070
11.1290
3493.59
8.0±0.3
AHC
B3CA
11.4800
26.4800
11.4800
3414.02
6.5±0.4
1050℃b
1000℃b
4.9±0.1
2.9±0.4
3.3±0.1
1.8±0.3
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AC
The standard deviations are calculated from five testing cathodes.
3.3
Morphological observation
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a—
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Table 2 The component phases, cell parameters, and the space charge-limited current density Jdiv of the 6:1:2 BCA using AC and AHC as precipitators.
The FESEM and EDS measurements were carried out to further investigate the effect of precipitants on the microstructure. Fig. 4 shows the FESEM images of the
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co-precipitation precursors obtained by using AC and AHC and the corresponding samples calcined at 1200 ℃, respectively. The EDS mapping of both precursors are
AC C
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shown in Fig. 5.
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Figure 4. The FESEM micrographs of the precursors (a and b) and calcined samples (c and
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d): (a) precipitated by AC; (b) precipitated by AHC; (c) 6:1:2 BCA precipitated by AC after
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calcined at 1200 ℃; (d) 6:1:2 BCA precipitated by AHC after calcined at 1200 ℃
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Figure 5. FESEM images of the precursors precipitated by AC (a) and AHC (e), and EDS mapping of Ba (b and f), Ca (c and g) and Al (d and h) elements.
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It can be seen from Fig. 4 (a) and (b) that the precursor precipitated by AC consists of some particles surrounded by silk-like crystals. Whereas, the precursor precipitated by AHC comprise broccoli-like spherical particles and the matrix. The matrix is amorphous because not any specific configuration can be observed, which is in accordance with the XRD result [see Fig. 3 (b)]. As depicted in Fig. 5 (b)~(d), Al can be observed in the silk-like phase. Therefore, the silk-like phase can be ascribed to the AACH and the remaining particles are of (Ba,Ca)CO3. In Fig. 5 (f)~(h), the 16
ACCEPTED MANUSCRIPT broccoli-like spherical particles are abundant with Ba and Ca elements, however, the matrix is abundant with Al. Accordingly, the broccoli-like particles are of (Ba,Ca)CO3 and the matrix is of Al3+-contained salt.
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Micrograph of the precursors precipitated by AC and AHC calcined at 1200 ℃ are also studies by FESEM, and the results are shown in Fig. 4 (c) and (d),
respectively. It can be seen that the grain boundaries can be observed clearly for both
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samples. Additionally, the calcined aluminate precipitated by AC at 1200 ℃
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exhibited much lower agglomeration and rounder in shape than the sample precipitated by AHC and calcined at the same temperature. 3.4
Electronic emission capability of ICDs
The applied voltage (U) dependencies of emission current density (J) of the ICDs
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impregnated with 6:1:2 type BCA using AC and AHC as precipitators measured at 1000, 1050 and 1130 ℃b (℃b is brightness temperature) with DC mode are shown in Fig. 5 (a) and (b), respectively. The space charge-limited current density in term of
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Jdiv can be determined from Fig. 5 and the results are listed in Table 2.
Figure 5. (Color online) The log J-log U plots of the impregnated 6:1:2 type BCA ICDs with impregnants by using AC and AHC as precipitators. Measurements were performed at 1000, 1050, and 1130 ℃b with DC mode. 17
ACCEPTED MANUSCRIPT The Jdiv of the ICDs impregnated with 6:1:2 type BCA using AC precipitant reach 8.0±0.3 A/cm2 at 1130℃b. However, the Jdiv of the cathodes impregnated with aluminate using AHC is 6.5±0.4 A/cm2 at 1130 ℃b, which is about 23 % lower than
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that of the AC ICDs. It is well known that the emission capacity of ICDs depends on several factors, such as the porosity of W matrix, barium concentration and phase
compositions of BCA impregnant [1]. Generally, the Jdiv is mainly determined by the
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barium content of the BCA impregnant apart from the density of pores of the tungsten
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matrix [32]. In this work, the barium content of 6:1:2 type BCA precipitated by AC is higher than that prepared by AHC precipitant, which is confirmed from the difference in the cell parameters (see Table 2). Therefore, the main reason of the difference in the emission capacity of the two kinds of ICDs in this work can reasonably be
4. Conclusions
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attributed to the difference in the barium content.
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In summary, the effects of different precipitants of AC and AHC on the structure of precursors and the calcined aluminates, as well as the thermionic emission capacity
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are systematically studied in this work. Our results indicate that the precursor precipitated by AC has the component phases of (Ba,Ca)CO3 particles and silk-like AACH crystals. In comparison, the precursor synthesized by AHC comprises broccoli-like (Ba,Ca)CO3 particles and the amorphous phase comprising Al3+. The two precursors exhibit similar thermal behaviors and phase evolution during calcining at different temperatures except for an extra peak attributed to the thermo-decomposition of AACH in the DSC curves. After calcining at 1200 ℃, both two aluminates 18
ACCEPTED MANUSCRIPT crystallized in the B3CA structure but have different barium content revealed from the cell parameters. DC current emission measurements present that the ICDs impregnated with the AC precursor aluminate possess current density of
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8.0 ± 0.3 A/cm2 at 1130 ℃b, which is greater than that of the ACH precursor BCA by about 23 %.The higher barium content in the former BCA is responsible for the
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superior emission capacity.
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Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51750110501).The authors thank Dr. Shengyi Yin from the Key Laboratory of High Power Microwave Sources and Technologies, Institute of Electronics, Chinese
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