materials Article
Effect of CaO on Phase Composition and Properties of Aluminates for Barium Tungsten Cathode Jinglin Li 1,2 , Jianjun Wei 1,2 , Yongbao Feng 1,2, * and Xiaoyun Li 1,2 1 2
*
College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China;
[email protected] (J.L.);
[email protected] (J.W.);
[email protected] (X.L.) Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, China Correspondence:
[email protected]; Tel.: +86-139-5162-3119
Received: 5 June 2018; Accepted: 5 August 2018; Published: 8 August 2018
Abstract: 6BaO·xCaO·2Al2 O3 (x = 0.8, 1.2, 1.6, 2, and 2.2) aluminates were synthesized via a liquid phase co-precipitation method. Effects of the molar amount of CaO on the phase of aluminates before and after melting and their hygroscopic phase, melting properties, environmental stability, evaporation, and emission properties were systematically studied. The results show that with the increase of the molar amount of CaO, the aluminates change from a mixture phase to a single phase of Ba3 CaAl2 O7 , and the diffraction peak shifts to a higher angle. The melted phase of the aluminates changed from a single phase to a mixed phase of Ba5 CaAl4 O12 and Ba3 CaAl2 O7 . Meanwhile, the comprehensive properties of the aluminates are improved. The weight gain of 6BaO·2CaO·2Al2 O3 aluminates is only 10.88% after exposure to air for 48 h; the pulse emission current density of barium tungsten cathodes impregnated with 6BaO·2CaO·2Al2 O3 aluminates in the porous tungsten matrix can reach 28.60 A/cm2 at 1050 ◦ C, and the evaporation rate is 2.52 × 10−10 g/(cm2 ·s). Keywords: aluminates; molar amount of CaO; phase composition; emission current; evaporation rate
1. Introduction As the main electron source for vacuum electronic devices, the impregnated barium-tungsten (Ba-W) cathode has the desirable characteristics of high emission current density, long lifetime, resistance to ion bombardment, and so on [1,2]. It is prepared by impregnating a molten electron emission salt into the porous tungsten matrix, followed by processing and coating with a film [3]. The composition of the electron emission source has an important effect on the emission performance and lifetime of the Ba-W cathode [4,5]. The most widely used electronic emission materials in Ba-W cathodes are aluminates. According to the different molar ratios of BaO, CaO and Al2 O3 in aluminates, they can be categorized as 612 (i.e., nBaO :nCaO :nAl2O3 = 6:1:2) [6,7], 411 [8], 532 [9], and 311 [10] aluminates. To improve the properties of aluminates for Ba-W cathodes, researchers have focused on the effects of the synthesis process [8,11,12], the phase composition [6,13], the scandium element doping [14–16], and porous tungsten matrix [17] on the properties of aluminates. C. Lai et al. [14,18,19] proposed the sputtering of a layer of unit or multi-element metal film on the surface of Ba-W cathode to prepare a coated cathode, which can improve its emission performance. However, there are fewer studies on the relationship between chemical composition, structure, and properties of aluminates. Ba and BaO are used as sources of electron emission in aluminates, and their molar amount has a direct effect on emission and evaporation of the cathode [20,21]. CaO cannot provide emission electrons, but it has an important influence on the environmental stability, melting characteristics, and phase composition of aluminates, which has an indirect effect on the emission and evaporation properties of aluminates for Materials 2018, 11, 1380; doi:10.3390/ma11081380
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Ba-W cathodes [22]. 6BaO·CaO·2Al2 O3 is used widely in electron emitting aluminates, but the effect of CaO content on its structure and properties has rarely been reported. In this paper, 6BaO·xCaO·2Al2 O3 (x = 0.8, 1.2, 1.6, 2, and 2.2) aluminates were prepared by a liquid phase method. Effects of the molar amount of CaO on the phase composition, melting characteristics, environmental stability of aluminates, and the performance of Ba-W cathodes were systematically studied. 2. Materials and Methods 2.1. Experimental Process In this paper, analytical purities of Ba(NO3 )2 (≥99.5%, Guo Yao Group Co., Ltd., Shanghai, China), Ca(NO3 )2 ·4H2 O (≥99.0%, Guo Yao Group Co., Ltd., Shanghai, China), and Al(NO3 )3 ·9H2 O (≥99.0%, Guo Yao Group Co., Ltd., Shanghai, China) were used as raw materials. The three nitrates were weighed at a molar ratio and formulated into a mixed salt solution and then placed in a water bath at 10 ◦ C. A mixture of (NH4 )2 CO3 (≥99.0%, Ling Feng Chemical Reagent Co., Ltd., Shanghai, China) and NH3 ·H2 O (Ling Feng Chemical Reagent Co., Ltd., Shanghai, China) [23] (W((NH4 )2 CO3 ) = 14.09%, W(25–28% NH3 ·H2 O) = 6.09%) was then rapidly added to the mixed salt solution while stirring until the pH = 8.5 [6]. Five kinds of 6BaO·xCaO·2Al2 O3 aluminate precursors were obtained after the reaction products were aged, washed by centrifugation, and dried. Each of the five precursor powders were compacted into Φ 15 mm × 30 mm cylindrical samples by cold isostatic pressing technology at 300 MPa for 90 s. The samples were placed in a high-purity alumina crucible and calcined in an air atmosphere at 1500 ◦ C for 2 h, followed by a cooling rate of 2 ◦ C per minute. The 6BaO·xCaO·2Al2 O3 aluminate powders were obtained by grinding sintered samples in an agate mortar, which were then vacuum-packed. 2.2. Performance Testing Phase identification of aluminates before and after melting and their hygroscopic phase were determined via X-ray powder diffractometer (XRD) (Rigaku, SmartLab, Tokyo, Japan) using Cu-Kα radiation and step scanning over a 2θ range from 10◦ to 90◦ , with a scan rate of 10◦ /min. Melting characteristics of aluminates pressed into a cylinder of Φ 8.5 mm × 7 mm were investigated by a high temperature real-time observation and test system (TOM-AC, CeDeD equipment R&D center, Dresden, Germany). The environmental stability of the aluminates at 25 ◦ C and 50% RH were measured while using a constant temperature and humidity chamber (LHS-100HC, Shanghai Aozhen Instrument Manufacturing Co., Ltd., Shanghai, China). The aluminates were impregnated into a tungsten matrix with a porosity of 25–26%. After processing and film mulching, an impregnated Ba-W cathode of Φ 3.4 mm × 1 mm was prepared. The evaporation rate of aluminates at 1050–1200 ◦ C were measured by the Baker method using an electronic injection analysis system (AD-NTECH, Yanhua Science and Technology Co., Ltd., Nanjing, China). The impregnated cathode was assembled into a water-cooled anode diode, and its pulse emission performance at 1050 ◦ C was tested after activation at 1200 ◦ C, using the cathode performance test system (YYC-10, Yanhua Technology Co., Ltd., Yangzhou, China), which the accuracy of ammeter is 0.01 A. 3. Results 3.1. Phase Composition of 6BaO·xCaO·2Al2 O3 Aluminates Figure 1 displays the XRD patterns of aluminates with different molar amounts of CaO; Figure 1b is a partial enlarged drawing of 2θ in the range of 29◦ to 32◦ . It could be seen from Figure 1a that the aluminate phase was a mixture of Ba3 CaAl2 O7 and Ba5 CaAl4 O12 when the molar amount of CaO was 0.8, and when the molar amount of CaO was more than 1.2, only a single Ba3 CaAl2 O7 phase was
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seen.From From Figure it could determined that, with increase of molar the molar amount of CaO, seen. Figure 1b,1b, it could be be determined that, with the the increase of the amount of CaO, the ◦ became wider and shifts to a higher angle. the diffraction peak of Ba CaAl O at 30–31 diffraction peak of Ba3CaAl became wider and shifts to a higher angle. 3 2O7 at2 30–31° 7
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Figure 6BaO· Figure1.1.X-ray X-raypowder powderdiffractometer diffractometer (XRD) (XRD) patterns patterns of of 6BaO ·xCaO· xCaO·2Al 2Al22OO3 3aluminates. aluminates.(a) (a)XRD XRD patterns in 10~90°; and, (b) Partial enlarged drawing in 29~32° of (a). ◦ ◦ patterns in 10~90 ; and, (b) Partial enlarged drawing in 29~32 of (a).
When x decreased from 2 to 1.2, the main crystal phases of 6BaO·xCaO·2Al 2O3 were all When x decreased from 2 to 1.2, the main crystal phases of 6BaO·xCaO·2Al O were all Ba3CaAl2O7 phase, and the corresponding decrease of Ca2+2+did not cause any change 2of 3the main Ba3 CaAl2 O7 phase, and the corresponding decrease of Ca did not cause any change of the main crystal phase structure of aluminates. According to the diffraction peak shifts to lower angles, the crystal phase structure of aluminates. According to the diffraction peak shifts to lower angles, absence of Ca2+ only caused lattice distortion, and the corresponding interplanar spacing increased. the absence of Ca2+ only caused lattice distortion, and the corresponding interplanar spacing increased. When the molar amount of CaO was 0.8, the structure of Ba3CaAl2O7 phase was unstable due to the When the molar amount of CaO was 0.8, the structure of Ba3 CaAl2 O7 phase was unstable due to the many Ca2+2+vacancies, and the second phase of Ba5CaAl4O12 was formed. When the molar amount of many Ca vacancies, and the second phase of Ba5 CaAl4 O12 was formed. When the molar amount of CaO increased from 2 to 2.2, the excess CaO had no effect on the phase composition of aluminates, CaO increased from 2 to 2.2, the excess CaO had no effect on the phase composition of2+aluminates, and the diffraction peak continued to shift to a higher angle, which suggested that Ca might be and the diffraction peak continued to shift to a higher angle, which suggested that Ca2+ might be filling filling in the lattice of the Ba3CaAl2O7 phase in the form of interstitial ions, so that the interplanar in the lattice of the Ba CaAl O7 phase in the form of interstitial ions, so that the interplanar spacing spacing decreased [24].3 The 2reason for the broadening of the diffraction peak might be that the decreased [24]. The reason for the broadening of the diffraction peak might be that the increase of CaO increase of CaO made the crystallization degree of 6BaO·xCaO·2Al2O3 aluminates lower, and the made the crystallization degree of 6BaO·xCaO·2Al2 O3 aluminates lower, and the grain size decreased. grain size decreased.
3.2. Melting Characteristic of 6BaO·xCaO·2Al2 O3 Aluminates 3.2. Melting Characteristic of 6BaO· xCaO· 2Al2O3 Aluminates Melting characteristics can determine the impregnation effect of aluminates—under the same Melting characteristics can determine the impregnation effect of aluminates—under the same impregnation conditions, aluminates with lower melting temperatures will have a higher impregnation impregnation conditions, aluminates with lower melting temperatures will have a higher rate. Figure 2 displays the melting characteristics of five kinds of aluminates. The initial melting impregnation rate. Figure 2 displays the melting characteristics of five kinds of aluminates. The initial temperature and the total melting temperature of the aluminates increased first and then decreased melting temperature and the total melting temperature of the aluminates increased first and then with the increase of the molar amount of CaO. decreased with the increase of the molar amount of CaO.
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Figure 2. Melting propertiesof of6BaO 6BaO· 2O3 aluminates. Figure 2. Melting properties ·xCaO· xCaO2Al ·2Al 2 O3 aluminates.
When the molar amount of CaO increased from 0.8 to 1.2, the initial melting and total melting
When the molar amount of CaO increased from 0.8 to 1.2, the melting and total melting temperature gradually increased [25], so that the 6BaO· 1.2CaO· 2Alinitial 2O3 could only be completely temperature increased [25], so that the 6BaO·of 1.2CaO ·2Al2was O3 could onlytransformed be completely melted atgradually 1560 °C. This was perhaps because the phase aluminates gradually ◦ C. This from the mixed phase of Ba 3CaAl2O 7 and Bathe 5CaAl 4O12 to single phase Ba3CaAl2Otransformed 7 as the molar from melted at 1560 was perhaps because phase ofthe aluminates wasofgradually amount of CaO increased. The mixed phase might be slightly eutectic, which led to the mixed phase of Ba3 CaAl2 O7 and Ba5 CaAl4 O12 to the single phase of Ba3 CaAl2lower O7 as initial the molar melting and total melting temperatures, while the melting temperature of the single phaseinitial amount of CaO increased. The mixed phase might be slightly eutectic, which led to lower Ba3CaAl2O7 was higher. Then, the melting temperature decreased with the increase of the molar melting and total melting temperatures, while the melting temperature of the single phase Ba3 CaAl2 O7 amount of CaO, which might be because the phase of aluminates was still dominated by the was higher. Then, the melting temperature decreased with the increase of the molar amount of CaO, Ba3CaAl2O7 phase when the molar amount of CaO was greater than 1.2. There were more defects in whichthe might be because the phase of aluminates was still dominated by the Ba CaAl2 O7 phase when crystals and the grain size decreases, thus lowering the melting temperature. 3 the molar amount of CaO was greater than 1.2. There were more defects in the crystals and the grain size decreases, the 2Al melting temperature. 3.3. Molten thus Phaselowering of 6BaO· xCaO· 2O3 Aluminates It is very important to analyze the molten phase of aluminates, which reacts with the tungsten matrix to form active substances, for the study of factors that are affecting the cathode properties. Five kindsimportant of aluminates were rapidly heated tophase 1690 °C then cooled after reacts 90 s. XRD patterns of It is very to analyze the molten of and aluminates, which with the tungsten molten aluminates with different amounts of CaO that are shown in Figure with Figure 3b matrix to form active substances, formolar the study of factors are affecting the3, cathode properties. ◦ Cofand displaying a partial enlarged drawingheated of 2θ into the1690 range 29° to 32°.cooled The structure and Five kinds of aluminates were rapidly then after 90 s. properties XRD patterns of five kinds of aluminates with different molar amounts of CaO were displayed in Table 1. of molten aluminates with different molar amounts of CaO are shown in Figure 3, with Figure 3b
3.3. Molten Phase of 6BaO·xCaO·2Al2 O3 Aluminates
displaying a partial enlarged Table drawing of 2θ in the range of 29◦ to 32◦ . The structure and properties of 1. Properties of 6BaO·xCaO·2Al2O3 aluminates. five kinds of aluminates with different molar amounts of CaO were displayed in Table 1. Mole Ratio of BaO:CaO:Al2O3 6:0.8:2
Mole Ratio of 6:1.2:2 BaO:CaO:Al 6:1.6:2 2 O3 6:2.0:2 6:2.2:2
Phase Composition of Aluminates Prepared at Table 1500 °C 1. forProperties 2h Ba3CaAl2O7 Ba5CaAl4O12 of Phase Composition Ba3CaAl2O7 Aluminates Prepared at Ba 3CaAl2O7 ◦C 1500 Ba for 2 h 3CaAl2O7 Ba3CaAl 2O7 Ba3 CaAl 2O 7
Phase Composition of Aluminates Melted at of 6BaO ·2Al 165 ·°xCaO C for 90 s 2 O3 Ba5CaAl4O12 Ba3CaAl2O7
Initial Melting Temperature/°C aluminates.
Phase Composition of Ba3CaAl2O7 Ba5CaAl4O12 Aluminates Melted at Ba3CaAl2O 7 Ba 5CaAl 4O 12 165 ◦ C for 90 s Ba3CaAl2O7 Ba5CaAl4O12 Ba3CaAl 7 Ba5CaAl 4O12 Ba25OCaAl 4 O12
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Total Melting Temperature/°C 1546
Initial Total 1550 Melting 1560 Melting ◦ C Temperature/ ◦C Temperature/ 1536 1546 1527 1527
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1535 1546 Ba5 CaAl4 O12 Ba3 CaAl2 O7 Combining Figure 3 and Table 1, the phase Ba composition 3 CaAl2 O7 of aluminates before and after melting 1550 1560 6:1.2:2 Ba3 CaAl2 O7 Ba5 CaAl was changed. When the molar amount of CaO was 2.0 or4 O 2.2, 12 the molten phases of aluminates were Ba 3 CaAl 2O 7 fused Ba3CaAl2O7 phase exhibited the Ba 3CaAl2O7 with a very small amount of Ba5CaAl 4O 12, and the 1536 1546 6:1.6:2 Ba3 CaAl2 O7 Ba CaAl O12molar amount of CaO was less than or 5 4 preferential growth of individual crystal planes. When the Ba3 CaAl2 O7 equal 4O12, 6:2.0:2to 1.6, the phase Bachanged 15272O7 and Ba5CaAl 1547 3 CaAl2 O7from Ba3CaAl2O7 to the coexistence of Ba3CaAl Ba5 CaAl4 O12 and the content of the Ba5CaAl4O12 phase was increasing. This indicated that the aluminates of the Ba3 CaAl2 O7 1537 6:2.2:2 Ba3 CaAl Ba 3CaAl2O7 phase, which were proportions, led to the 2 O7 unstably synthesized with different CaO1527 Ba CaAl O 6:0.8:2
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Combining Figure 3 and Table 1, the phase composition of aluminates before and after melting was changed. When the molar amount of CaO was 2.0 or 2.2, the molten phases of aluminates were Ba3 CaAl2 O7 with a very small amount of Ba5 CaAl4 O12 , and the fused Ba3 CaAl2 O7 phase exhibited
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the preferential growth of individual crystal planes. When the molar amount of CaO was less than or equal to 1.6, the phase changed from Ba3 CaAl2 O7 to the coexistence of Ba3 CaAl2 O7 and Ba5 CaAl4 O12 , and the content of the Ba5 CaAl4 O12 phase was increasing. This indicated that the aluminates of the Materials Ba3 CaAl which were unstably synthesized with different CaO proportions, 2 O11, 7 phase, 2018, x FOR PEER REVIEW 5 ofled 10 to the deficiency or excess of Ca2+ due to the vacancies or the interstitials of CaO, so the phase separation deficiency or the excess of Ca2+ to temperature the vacancies melting or the interstitials of CaO, so the phase separation would occur in process ofdue high and cooling. would occur in the process of high temperature melting and cooling.
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Figure 3. XRD patterns of molten 6BaO·xCaO·2Al2O3 aluminates; (a) XRD patterns in 10~90°; and, (b)
Figure 3. XRD patterns of molten 6BaO·xCaO·2Al2 O3 aluminates; (a) XRD patterns in 10~90◦ ; Partial enlarged drawing in 29~32° of (a). and, (b) Partial enlarged drawing in 29~32◦ of (a). 3.4. Environmental Stability of 6BaO· xCaO· 2Al2O3 Aluminates
3.4. Environmental Stability of 6BaO·xCaO·2Al2 O3 Aluminates
The aluminates exposed to air can react with CO2, water vapor, and so on, and the macroscopic evidence for this isexposed the increase in mass. The with degree of2moisture absorption andon, COand 2 absorption of The aluminates to air can react CO , water vapor, and so the macroscopic aluminates can be judged from their weight gain rate [26]. Aluminates with low weight gain and high evidence for this is the increase in mass. The degree of moisture absorption and CO2 absorption of environmental ensure consistency and[26]. stability of the products in the cathode aluminates can bestability judgedwill from theirthe weight gain rate Aluminates with low weight gain and production process. Figure 4 shows the weight gain curves of aluminates with different molar high environmental stability will ensure the consistency and stability of the products in the cathode amounts of CaO over 48 h. As illustrated in Figure 4, when the molar amount of CaO in aluminates production process. Figure 4 shows the weight gain curves of aluminates with different molar amounts was increased from 0.8 to 2.0, the weight gain rate of aluminates gradually decreased from 15.89% to of CaO over 48 h. As illustrated in Figure 4, when the molar amount of CaO in aluminates was 10.88% for 48 h, and the environmental stability gradually improved [5], which might be related to increased from 0.8 to 2.0, structure the weight rate of aluminates gradually from 15.89% to the perfection of crystal andgain the formation of single-phase Ba3CaAldecreased 2O7. When the molar 10.88% for 48 h, and the environmental stability gradually improved [5], which might be related to the amount of CaO was 2.2, the weight gain rate of aluminates increased, and the environmental stability 2+ perfection crystal structure and the formation single-phase becameof worse, which might be related to the Ca of entering into theBa interstitial ofthe Ba3molar CaAl2Oamount 7 3 CaAl2 Oposition 7 . When and was the structure becoming toorate loose. From another point of and view,the it environmental could be deduced that the of CaO 2.2, the weight gain of aluminates increased, stability became 2+ entering aluminates of single Bato 3CaAl 7 had high environmental stability. position At the same time, the slope worse, which might bephase related the2O Ca into the interstitial of Ba 3 CaAl2 O7 and the of the weight gain curve decreased gradually with the prolongation of storage time, which meant structure becoming too loose. From another point of view, it could be deduced that the aluminates of that the ability of the aluminates to absorb moisture and CO2 gradually became saturated. For single phase Ba3 CaAl2 O7 had high environmental stability. At the same time, the slope of the weight 6BaO·xCaO·2Al2O3 aluminates, the proper amount of CaO could increase the stability of aluminates gain curve decreased gradually with the prolongation of storage time, which meant that the ability to a certain extent.
of the aluminates to absorb moisture and CO2 gradually became saturated. For 6BaO·xCaO·2Al2 O3 aluminates, the proper amount of CaO could increase the stability of aluminates to a certain extent.
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Figure 4. Weight gain of 6BaO·xCaO·2Al2O3 aluminates exposed to the air for different time. Figure ·xCaO ·2Al O33aluminates aluminatesexposed exposedto tothe theair airfor fordifferent differenttime. time. Figure 4. 4. Weight Weight gain gain of of 6BaO 6BaO· xCaO· 2Al22O
The XRD patterns of the aluminates that were exposed to air for 48 h are shown in Figure 5. XRD patterns patterns of of the the aluminates aluminates that that were were exposed exposed to to air air for for 48 48 hh are are shown shown in in Figure Figure 5. 5. The XRD When compared with Figures 1 and 5, it could be seen that the diffraction peaks of aluminates aluminates When compared with Figures 1 and 5, it could be seen that the diffraction peaks of aluminates appeared to broaden after moisture absorption and weight gain, and the reaction of aluminates with appeared to broaden after moisture absorption and weight gain, and the reaction of aluminates with H2O and CO2 in air resulted in the gradual disappearance of Ba3CaAl2O7 phase structure and the O and and CO CO2 in in air air resulted in the gradual O7 phase phase structure structure and and the H22O gradual disappearance disappearance of Ba Ba33CaAl22O formation of 2new phases, which included Ca(OH) 2, Ba7Al2O10, and other 7unknown phases [26,27]. formation of new phases, which included Al22O O1010, ,and andother otherunknown unknownphases phases[26,27]. [26,27]. formation included Ca(OH) Ca(OH) 2, Ba77Al Three characteristic diffraction peaks of Ba3CaAl2O27 phase disappeared at 29~30°◦ due to the serious phase disappeared disappeared at 29~30° 29~30 due due to to the the serious Three characteristic diffraction peaks peaks of of Ba Ba33CaAl22O77 phase hygroscopic absorption of aluminates with a molar ratio of 6:0.8:2. hygroscopic absorption of aluminates with a molar ratio of 6:0.8:2. Ba CaAl O Ba CaAl O Ca(OH) Ba3CaAl2O7 Ba 5 4 12 2 3 2 7 5CaAl4O12 Ca(OH)2 Ba Al O Unknown phase Ba7Al2O10 ■ Unknown phase 10 ■ 7 2
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50 60 70 80 90 50 60 70 80 90 2/(°) 2/(°) Figure 5. XRD patterns of 6BaO·xCaO·2Al2O3 aluminates exposured to the air for 48 h. Figure 5. 5. XRD XRD patterns patterns of of 6BaO 6BaO· xCaO· 2Al22O Figure ·xCaO ·2Al O33aluminates aluminatesexposured exposuredto tothe theair airfor for48 48h. h.
3.5. Emission Performance of 6BaO· xCaO· 2Al2O3 Aluminates 3.5. Emission Performance of 6BaO· xCaO· 2Al2O3 Aluminates
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The pulse emission curves of aluminate-Ba-W cathodes with different molar amounts of CaO The pulse emission curves of aluminate-Ba-W cathodes with different molar amounts of CaO are shown in Figure 6. It could be seen from the diagram that the pulse emission current densities of are shown in Figure 6. It could be seen from the diagram that the pulse emission current densities of ◦ five kinds of aluminate-Ba-W cathode with CaO molar amounts of 0.8, 1.2, 1.6, 2, and 2.2 at 1050 C five kinds of aluminate-Ba-W cathode with CaO molar amounts of 0.8, 1.2, 1.6, 2, and 2.2 at 1050 °C 2 , 26.06 A/cm2 , 26.06 A/cm2 , 28.60 A/cm2 , and 20.08 A/cm2 , respectively. When the were were 24.0924.09 A/cm 2, 26.06 A/cm2, 26.06 A/cm2, 28.60 A/cm2, and 20.08 A/cm2, respectively. When the A/cm molarmolar amount of CaO increased, currentdensity density aluminates increased first and amount of CaO increased,the thepulse pulseemission emission current of of aluminates increased first and then then decreased; among them, 6BaO ·2CaO ·2Al O33aluminate aluminate was highest. decreased; among them, 6BaO· 2CaO· 2Al22O was thethe highest.
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Figure 6. Pulse emission curvesofof6BaO 6BaO· xCaO· barium tungsten cathodes. Figure 6. Pulse emission curves ·xCaO ·2Al 2Al22OO3 3aluminates aluminates barium tungsten cathodes.
Combined with the molten phase results of 6BaO·xCaO·2Al2O3 aluminates, it could be seen that
Combined with the molten phase results of 6BaO·xCaO·2Al O3 aluminates, it could be seen as the molar amount of CaO increased from 0.8 to 2.0, the content of2 BaO decreased gradually, but that as molarproperty amountwas of improved, CaO increased 0.8 to 2.0, content of of BaO decreased gradually, its the emission whichfrom indicated that thethe molten phase aluminates played a but its emission property was improved, which indicated that the molten phase of aluminates played decisive role in its emission performance. There was a very small amount of Ba5CaAl4O12 in the a decisive in of its 6BaO· emission performance. There wasthe a very amount of was Ba5 CaAl moltenrole phase 2CaO· 2Al2O3 aluminates, while mainsmall crystalline phase Ba3CaAl O7in , the 4 O212 butphase the second phase was·2Al present in higher amounts in other different molten of 6BaO ·2CaO while the main aluminates crystallinewith phase was Bamolar 2 O3 aluminates, 3 CaAl2 O7 , amounts ofphase CaO [28]. the Ba 2O7amounts phase might have higher reactivity W [6], molar producing but the second wasThus, present in3CaAl higher in other aluminates withwith different amounts more Ba and BaO, leading to the Ba-W cathode having a higher emission current density. However, of CaO [28]. Thus, the Ba3 CaAl2 O7 phase might have higher reactivity with W [6], producing more Ba the emission property of aluminates decreased when the molar amount of CaO was 2.2, which might and BaO, leading to the Ba-W cathode having a higher emission current density. However, the emission be due to the decrease of BaO content in the aluminates and the presence of more of the non-reactive property of aluminates decreased when the molar amount of CaO was 2.2, which might be due to second phase in the molten product. Therefore, the reaction activity of the melt product and tungsten the decrease of BaO content in the aluminates and the presence of more of the non-reactive second matrix decreased, which led to the decrease of the emission property. phase in the molten product. Therefore, the reaction activity of the melt product and tungsten matrix decreased, which led toofthe decrease of 2the emission property. 3.6. Evaporation Rate 6BaO· xCaO· 2Al O3 Aluminates The evaporation rate curve of Ba-W cathodes that were prepared with five kinds of aluminates 3.6. Evaporation Rate of 6BaO·xCaO·2Al2 O3 Aluminates
at 1050 °C are shown in Figure 7. It could be clearly seen that the average evaporation rate of
impregnated aluminate-Ba-W increase of the with molarfive amount of of CaO [25], The evaporation rate curvecathodes of Ba-Wdecreased cathodeswith thatthe were prepared kinds aluminates ◦ which indicated that this increase could reduce the evaporation of electronically active substances at 1050 C are shown in Figure 7. It could be clearly seen that the average evaporation rate of and prolong the lifetime of cathodes the cathode at working temperatures. evaporation rate of barium impregnated aluminate-Ba-W decreased with the increaseThe of the molar amount of CaO [25], tungsten cathodes that were impregnated with 6BaO· 2CaO· 2Al 2O3 aluminates in the porous tungsten which indicated that this increase could reduce the evaporation of electronically active substances and matrix was only 2.52 × 10−10 g/(cm2·s). The mass ratio of Ba and BaO, as active vaporizers, determined prolong the lifetime of the cathode at working temperatures. The evaporation rate of barium tungsten the evaporation rate of the aluminates and usually PBa/PBaO > 1 [29,30], so the main evaporative specie cathodes that were impregnated with 6BaO·2CaO·2Al2 O3 aluminates in the porous tungsten matrix of the Ba-W cathode was Ba. The molten phase of aluminates tended towards a single phase of was only 2.52 × 10−10 g/(cm2 ·s). The mass ratio of Ba and BaO, as active vaporizers, determined Ba3CaAl 2O7 with the increase of the molar amount of CaO, which indicated that the Ba3CaAl2O7 phase the evaporation ratelikely of the aluminates and usually /PBaO > 1 [29,30], thecompared main evaporative aluminates were to produce a BaO emitter with P a Ba lower evaporation rate so when with specie of the Ba-W cathode was Ba. The molten phase of aluminates tended towards a single the Ba5CaAl4O12 phase aluminates, while Ba5CaAl4O12 phase aluminates easily reacted with W tophase a 7high evaporation rateofofthe Ba. molar amount of CaO, which indicated that the Ba3 CaAl2 O7 of Baproduce with the increase 3 CaAl2 O phase aluminates were likely to produce a BaO emitter with a lower evaporation rate when compared with the Ba5 CaAl4 O12 phase aluminates, while Ba5 CaAl4 O12 phase aluminates easily reacted with W to produce a high evaporation rate of Ba.
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Evaporation 1010 (g(· cm ss rate/ Evaporationrate/ g· cm)· )·
-1 -1
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6.0 6.0 5.5 5.5 5.0 5.0 4.5 4.5 4.0 4.0 3.5 3.5 3.0 3.0 2.5 2.5 2.0 2.0
0.8 1.2 1.6 2.0 0.8 Mole 1.2 1.6CaO/mol 2.0 numbers of Mole numbers of CaO/mol
Figure 7. Average Average evaporationrate rate diagramof of cathodesfilled filled with6BaO 6BaO· xCaO· 2Al2O33 aluminates. Figure ·xCaO ·2Al aluminates. Figure 7. 7. Average evaporation evaporation rate diagram diagram ofcathodes cathodes filledwith with 6BaO· xCaO· 2Al22O O3 aluminates.
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Evaporation 10 cm ss rate/ Evaporationrate/ 10 (g(· g· cm)· )·
-1-1
Figure 8 shows the evaporation evaporation rate of of the Ba-W Ba-W cathode prepared prepared by6BaO 6BaO· 2CaO· 2Al 2O3 ·2CaO ·2Al Figure 8 shows the the evaporation rate rate of the the Ba-W cathode cathode prepared by by 6BaO· 2CaO· 2Al22O O33 aluminates at different temperatures. temperatures. The evaporation rate of the cathode increased with the increase aluminates at different The evaporation rate of the cathode increased with the increase of working working temperature temperature [31]. because the high temperature temperature could of of [31]. This This was was because the high could promote promote the the reaction reaction of aluminate with W and and thus thus produced produced more more electron electron emitters. Therefore, Therefore, the the working working temperature temperature aluminate aluminate with W electron emitters. emitters. should be as low as as possible when when the the current current density density was was satisfied. satisfied. should be as low possible satisfied.
40 40 35 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0 1050 1050
1100 1100
T/℃ T/℃
1150 1150
1200 1200
Figure 8. Average evaporation rate of cathodes filled with 6:2.0:2 aluminates at different temperatures. Figure Figure 8. 8. Average Average evaporation evaporation rate rate of of cathodes cathodes filled filled with with 6:2.0:2 6:2.0:2 aluminates aluminates at at different different temperatures. temperatures.
4. Conclusions 4. 4. Conclusions Conclusions Five kinds of 6BaO·xCaO·2Al2O3 (x = 0.8, 1.2, 1.6, 2, 2.2) aluminates were prepared by liquid coFive kinds of 6BaO· 2Al 3 (x = 0.8, 1.2, 1.6, 2, 2.2) aluminates were prepared by liquid coFive kinds ofphases 6BaOxCaO· ·xCaO ·2Al2O (x = 0.8, 1.2, 1.6,melting 2, 2.2) aluminates were prepared bymelting liquid 2 O3 before precipitation. The of aluminates and after and their hygroscopic phase, precipitation. The phases of aluminates beforebefore and after melting and their hygroscopic phase, melting co-precipitation. The phases of aluminates and after melting and their hygroscopic phase, characteristics, environmental stability, evaporation, and emission were studied. The main results are characteristics, environmental stability, evaporation, and emission studied. main results are melting characteristics, environmental stability, evaporation, and were emission wereThe studied. The main as follows: as follows: results are as follows: (1) Except for 6BaO·0.8CaO·2Al2O3, the aluminates are a mixed phase of Ba5CaAl4O12 and (1) Except 6BaO· mixed phase phase of of Ba Ba55CaAl CaAl44O O12 12 and and (1) Except for for 6BaO ·0.8CaO· 0.8CaO·2Al 2Al2OO3,3 , the the aluminates aluminates are are aa mixed Ba3CaAl2O7. The increase of the2 molar amount of CaO has little effect on the phase composition Ba 3 CaAl 2 O 7 . The increase of the molar amount of CaO has little effect on the phase composition Ba CaAl2 O7 . The increase of the molar amount of CaO has little effect on the phase composition of 3aluminates, but it can increase the interplanar spacing. The molten phase of aluminates of increase the the interplanar interplanar spacing. spacing. The aluminates of aluminates, aluminates, but but it it can can increase The molten molten phase phase of of aluminates gradually changed from Ba5CaAl4O12 to the mixture phase of Ba5CaAl4O12 and Ba3CaAl2O7 as the gradually 12 toto the mixture phase of Ba 5CaAl 4O12 and Ba 3Ba CaAl 2O7 as the gradually changed changedfrom fromBa Ba55CaAl CaAl4O O the mixture phase of Ba CaAl O and CaAl 5 4 12 3 2 O7 as molar amount of CaO increased. 4 12 molar amount of CaO increased. the molar amount of CaO increased. (2) With the increase of the molar amount of CaO, the initial melting temperature and the total (2) With increase the molar amount of the melting temperature and total (2) With the thetemperature increase of of of the6BaO· molar amount of3 CaO, CaO, the initial initial melting and the the total melting xCaO· 2Al2O aluminates increased firsttemperature and then decreased. The melting temperature of 6BaO· xCaO· 2Al 2O3 aluminates increased first and then decreased. The melting temperature ·xCaOcan ·2Albe increased first andthe then decreased. 2 Oimproved 3 aluminates environmental stabilityofof6BaO aluminates by properly increasing molar amount environmental stability of aluminates can be improved by properly increasing the molar amount of CaO, and the lowest 48 h weight gain rate of 6BaO·2CaO·2Al2O3 aluminates of single-phase of CaO, and the lowest 48 h weight gain rate of 6BaO·2CaO·2Al2O3 aluminates of single-phase
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The environmental stability of aluminates can be improved by properly increasing the molar amount of CaO, and the lowest 48 h weight gain rate of 6BaO·2CaO·2Al2 O3 aluminates of single-phase Ba3 CaAl2 O7 is 10.88%. However, when the molar amount of CaO is more than 2.2, the environmental stability of aluminates becomes worse and the hygroscopicity increases due to lattice distortion that is caused by Ca2+ . After exposure for 48 h, the diffraction peaks of aluminates are broadened to indicate the presence of Ca(OH)2 , Ba7 Al2 O10 , and unknown phases. The emission properties of aluminates can be improved with the increase of the molar amount of CaO. The maximum pulse emission current density of Ba-W cathode prepared by 6BaO·2CaO·2Al2 O3 aluminates is 28.6 A/cm2 at 1050 ◦ C However, when the molar amount of CaO exceeds 2.2, the emission performance of aluminates decreases. CaO can effectively reduce the evaporation rate of aluminate-Ba-W cathodes at working temperatures.
Author Contributions: Conceptualization, J.L., J.W., Y.F. and X.L.; Data curation, J.L., J.W., Y.F. and X.L.; Formal analysis, J.L., J.W., Y.F. and X.L.; Investigation, J.L., J.W. and Y.F.; Methodology, J.L., J.W. and Y.F.; Project administration, Y.F.; Supervision, Y.F.; Validation, Y.F.; Writing—original draft, J.L., and J.W. Drafting of manuscript: J.L., and J.W.; All of test: J.L., J.W., and X.L.; Planning and supervision of the research: Y.F., and X.L. Funding: This research was funded by National Natural Science Foundation of China No. 5130707. Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 51307079). Conflicts of Interest: The authors declare no conflict of interest.
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