六硼化镧粉体的制备及其烧结性能研究
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摘要
六硼化镧(LaB_6)材料具有电导率高、稳定性好、高温下蒸发率低、发射电流密度大等优异性能,已在电子显微镜、电子束焊接、放电管等需要大发射电流的领域获得应用。因其所具有的优异特性以及广泛的应用领域,大量科研机构都陆续开展了LaB_6材料的研究。然而,为了制备高性能的LaB_6多晶材料,首先需要制得纯度高、粒度分布均匀的LaB_6粉末。因此,本文旨在研究纯度高、烧结性能良好、粒度分布均匀的LaB_6粉末的低成本制备技术,为制备高性能LaB_6多晶体及其复合材料奠定基础。
以La_2O_3(D50<0.2μm)和B4C(D50=3.70μm)为原料,在常压下合成了LaB_6粉末。计算了常压合成LaB_6的热力学条件,采用XRD、SEM、激光粒度分析表征了不同温度、保温时间和不同粒径B4C原料合成的LaB_6粉末的物相组成、颗粒形貌和粒度分布,探讨了LaB_6粉末的合成反应机理。结果表明,常压下1650℃保温2h的产物经酸洗后,能得到纯度为99.22%、平均粒径为15.86μm、粒度分布均匀、具有立方体结构的LaB_6粉末。当La_2O_3颗粒尺寸远小于B4C时,LaB_6首先在B4C表面生成,随温度升高和保温时间延长,未反应的La_2O_3和LaBO3通过LaB_6壳不断扩散到B4C核表面直至反应完全。温度和保温时间对合成LaB_6颗粒的影响没有B4C原料明显,合成LaB_6粉末的初始形貌和尺寸主要取决于反应物B4C原料。
本文采用热压烧结技术制备了LaB_6多晶体,系统研究了烧结温度、保温时间、外加压力和粉体粒度对LaB_6多晶体性能的影响,初步探讨了LaB_6多晶体的热压烧结机理。研究结果表明:以自制的纯度达99.22%、平均粒径为15.86μm的LaB_6粉末为原料,在2050℃、30MPa条件下保温1h,制备的LaB_6多晶体的相对致密度达到96.32%,抗弯强度和显微硬度分别达到153.2MPa和16.27GPa。烧结温度、保温时间和外加压力的提高,有利于LaB_6多晶体的烧结致密化,但过高的烧结和延长保温时间均不利于材料的进一步致密化,反而会使晶粒异常长大,材料性能急剧下降。采用较小粒径的LaB_6粉末有利于增加致密化烧结速率,也有利于减小气孔尺寸和气孔率,能有效提高样品的烧结密度。
Lanthanum hexaboride has been receiving more and more attentions as a high performance material because of its unique properties, such as high electric conductivity, good chemical stability, low rate of evaporation at high temperature and high emission current density. LaB_6 is widely used as cathode parts in demanding huge emission current field instrument, such us electron microscope, electron beam welding industry, discharge tube industry and so on. Due to its special properties and broad application prospect, the type of materials research have been carried out by many institutions. However, in order to preparation high performance LaB_6 polycrystalline, high purity and uniform particle size distribution of LaB_6 powder is been needed at first. So the low cost preparation technology of high purity and good sintering performance of LaB_6 powder were investigated in this paper, which may establish the academic base for the preparation of high performance LaB_6 polycrystalline and composites.
The lanthanum hexaboride powders (LaB_6) were synthesized under atmospheric pressure by using La_2O_3 (D50<0.2μm) and B4C (D50=3.70μm) powder as the precursors. The reaction thermodynamic data for preparing LaB_6 was calculated. The effects of temperature, holding time and different particlesize of B4C on the phase composition, morphology, particle size and distribution of LaB_6 powders were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM) and Laser particle size anslyzer. The reaction mechanism was also investigated. Results showed that LaB_6 powders can be obtained under atmospheric pressure at 1650℃for 2h with 99.22% purity, D50=15.86μm, uniform particle size distribution and cube structure. When the particle size of La_2O_3 powder is much smaller than that of B4C particles, the growth of LaB_6 starts on the surface of B4C particles, then the residual La_2O_3 and LaBO3 will diffuse through LaB_6 shell onto B4C core until the reaction is completely. The influence of temperature and holding time on the synthesis of LaB_6 powder were not obviously than B4C raw materials, The initial shape and particle size of LaB_6 powders were depended on that of B4C particles.
The LaB_6 polycrystalline was produced by using hot-pressing method. The influence of temperature, holding time, hot-pressure and powder particle-size on the LaB_6 polycrystalline performance were system characterized. The LaB_6 polycrystalline hot-pressing sintering mechanism was also investigated. Results show that high performance LaB_6 polycrystalline obtained under 30MPa hot-pressure, at 2050℃for 1h by using self-made LaB_6 powder with 99.22% purity and D50=15.86μm, which relative density reached 96.32%, bending strength and microhardness achieved respectively 153.2MPa and 16.27GPa. With the increasing of temperature, holding time and hot pressure are in favor of LaB_6 polycrystalline sintering densification. However, exorbitant sintering temperature and prolong holding time would lead to grain abnormal grow and material properties sharply decline, which is disadvantage of materials further densification. It is benefit of increasing sintering densification rate, decreasing the stomata size and porosity by using smaller particle-size LaB_6 powders as sintering materials.
以La_2O_3(D50<0.2μm)和B4C(D50=3.70μm)为原料,在常压下合成了LaB_6粉末。计算了常压合成LaB_6的热力学条件,采用XRD、SEM、激光粒度分析表征了不同温度、保温时间和不同粒径B4C原料合成的LaB_6粉末的物相组成、颗粒形貌和粒度分布,探讨了LaB_6粉末的合成反应机理。结果表明,常压下1650℃保温2h的产物经酸洗后,能得到纯度为99.22%、平均粒径为15.86μm、粒度分布均匀、具有立方体结构的LaB_6粉末。当La_2O_3颗粒尺寸远小于B4C时,LaB_6首先在B4C表面生成,随温度升高和保温时间延长,未反应的La_2O_3和LaBO3通过LaB_6壳不断扩散到B4C核表面直至反应完全。温度和保温时间对合成LaB_6颗粒的影响没有B4C原料明显,合成LaB_6粉末的初始形貌和尺寸主要取决于反应物B4C原料。
本文采用热压烧结技术制备了LaB_6多晶体,系统研究了烧结温度、保温时间、外加压力和粉体粒度对LaB_6多晶体性能的影响,初步探讨了LaB_6多晶体的热压烧结机理。研究结果表明:以自制的纯度达99.22%、平均粒径为15.86μm的LaB_6粉末为原料,在2050℃、30MPa条件下保温1h,制备的LaB_6多晶体的相对致密度达到96.32%,抗弯强度和显微硬度分别达到153.2MPa和16.27GPa。烧结温度、保温时间和外加压力的提高,有利于LaB_6多晶体的烧结致密化,但过高的烧结和延长保温时间均不利于材料的进一步致密化,反而会使晶粒异常长大,材料性能急剧下降。采用较小粒径的LaB_6粉末有利于增加致密化烧结速率,也有利于减小气孔尺寸和气孔率,能有效提高样品的烧结密度。
Lanthanum hexaboride has been receiving more and more attentions as a high performance material because of its unique properties, such as high electric conductivity, good chemical stability, low rate of evaporation at high temperature and high emission current density. LaB_6 is widely used as cathode parts in demanding huge emission current field instrument, such us electron microscope, electron beam welding industry, discharge tube industry and so on. Due to its special properties and broad application prospect, the type of materials research have been carried out by many institutions. However, in order to preparation high performance LaB_6 polycrystalline, high purity and uniform particle size distribution of LaB_6 powder is been needed at first. So the low cost preparation technology of high purity and good sintering performance of LaB_6 powder were investigated in this paper, which may establish the academic base for the preparation of high performance LaB_6 polycrystalline and composites.
The lanthanum hexaboride powders (LaB_6) were synthesized under atmospheric pressure by using La_2O_3 (D50<0.2μm) and B4C (D50=3.70μm) powder as the precursors. The reaction thermodynamic data for preparing LaB_6 was calculated. The effects of temperature, holding time and different particlesize of B4C on the phase composition, morphology, particle size and distribution of LaB_6 powders were characterized by X-ray diffraction(XRD), scanning electron microscope(SEM) and Laser particle size anslyzer. The reaction mechanism was also investigated. Results showed that LaB_6 powders can be obtained under atmospheric pressure at 1650℃for 2h with 99.22% purity, D50=15.86μm, uniform particle size distribution and cube structure. When the particle size of La_2O_3 powder is much smaller than that of B4C particles, the growth of LaB_6 starts on the surface of B4C particles, then the residual La_2O_3 and LaBO3 will diffuse through LaB_6 shell onto B4C core until the reaction is completely. The influence of temperature and holding time on the synthesis of LaB_6 powder were not obviously than B4C raw materials, The initial shape and particle size of LaB_6 powders were depended on that of B4C particles.
The LaB_6 polycrystalline was produced by using hot-pressing method. The influence of temperature, holding time, hot-pressure and powder particle-size on the LaB_6 polycrystalline performance were system characterized. The LaB_6 polycrystalline hot-pressing sintering mechanism was also investigated. Results show that high performance LaB_6 polycrystalline obtained under 30MPa hot-pressure, at 2050℃for 1h by using self-made LaB_6 powder with 99.22% purity and D50=15.86μm, which relative density reached 96.32%, bending strength and microhardness achieved respectively 153.2MPa and 16.27GPa. With the increasing of temperature, holding time and hot pressure are in favor of LaB_6 polycrystalline sintering densification. However, exorbitant sintering temperature and prolong holding time would lead to grain abnormal grow and material properties sharply decline, which is disadvantage of materials further densification. It is benefit of increasing sintering densification rate, decreasing the stomata size and porosity by using smaller particle-size LaB_6 powders as sintering materials.
引文
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[3] Shigeki Otani, Yoshio Ishizawa. Thermionic emission properties of boron-rich LaB6, and CeB6, crystal cathodes. Journal of Alloys and Compounds, 1996, 245 (1-2): 18-20
[4] K Takahashi, S Kunii. Single crystal growth and properties of incongruently melting TbB6, DyB6, HoB6, YB6. Journal of Solid State Chemistry, 1997, 133(1): 198-200
[5] Yu Paderno, V Paderno, V Filippov. Some peculiarities of eutectic crystallization of LaB6(Ti, Zr)B2 Alloys. Journal of Solid State Chemistry, 2000, 154(1): 165-167
[6] M.E Schlesinger, P.K Liao, K.E Spear. The B-La (Boron-Lanthanum) system. Journal of Phase Equilibria, 1999, 20(1): 73-78
[7]林祖伦.射频电子枪中LaB6阴极的研究.强激光与粒子束, 1997, 9(4): 591-595
[8] Masaki Kuno, Takeo Oku, Katsuaki Suganuma. Synthesis of boron nitride nanotubes and nanocapsules with LaB6. Diamond and Related Materials 2001, 10 (3-7): 1231-1234
[9] J.M.Lafferty. Boride Cathodes. Journal Applied Physics. 1951, 22(3): 299-309
[10] A.N Broers. Some experimental and estimated characteristics of the lanthanum hexaboride rod cathode electron gun. Journal of Physics E:Scientific Instrument, 1969, (2): 273-276
[11] A.N Broers. Electron gun using long-Life lanthanum hexaboride cathode. Journal of Applied Physics. 1967, 38(4): 1991-1992
[12]金晓,刘锡三.单晶LaB6热阴极稳定性研究. 1995, 7(4): 555-560
[13]郑树起,阂光辉,邹增大,等.硼热还原法制备LaB6粉末.硅酸盐学报, 2001, 29(2): 128-131
[14]韩建德,王衍章,郑树起.电子束焊机六硼化镧阴极发射性能研究.山东工业大学学报, 2001, 31(4): 313-318
[15] L.W Swanson, M.A Gesley, P.R Davis. Crystallographic dependence of the work function and volatility of LaB6. Surface Science, 1981, 107(1): 263-289
[16] Taran A, S Plankovskyy, et al. High-current-density cathodes based on barium hafnate with tungsten: vacuumnce. Electronics Conference. Kirakyushu. IEEE International, 2007: 1-2
[17] V Paderno, Y Paderno, V Britun. Features of the real structure of lanthanum hexaboride single crystals. Journal of Alloys and Compounds, 1995, 219(1-2): 228-231
[18] K Flachbart, M Reiers, S Mloka, et al. Thermal conductivity of LaB6: the role of phonons. Physica B: Condensed Matter, 1999, 263-264(3): 749-751
[19] I Batko, M Batkova, K Flachbart, et al. Electrical resistivity and super- conductivity of LaB6 and LuB12. Journal of Alloys and Compounds, 1995, 217(2): 1-3
[20] S S Ordan yan, Yu B Paderno, I K Khoroshilova, et al. Interaction in the LaB6- ZrB2 System. Powder Metall, 1983, 19(11): 87-90
[21] B Galanov, V Kartuzov, Y Kartuzov, et al. Calculation of mechanical properties of hypothetic eutectic nanocomposites LaB6-MeB2. Journal of the European Ceramic Society, 2008, 28(12): 2331-2335
[22] V L Yupko, P A Verkhovodov, V V Morozov, et al. Wetting and contact reactions in the LaB6-Co and LaB6-Ni Systems. Powder Metallurgy and Metal Ceramic, 1981, 20(3): 207-210
[23] P H Schmidt, L D Longinotti, D C Joy, et al. Design and optimization of directly heated LaB6 cathode assemblies for electron-beam instruments. Journal of Vacuum Science and Technology, 1978, 15(4): 1554-1560
[24] Shintake Tumoru, Ohba Kunio, Matoba Masaru, et al. Pierce-type electron gun with a large concave LaB6 cathode. Japanese Journal of Applied Physics, 1981, 20(2): 341-345
[25] D M Goebel, Y Hirooka, T A Sketchley. Large-area lanthanum hexaboride electron emitter. Review of Scientific Instruments, 1985, 56(9): 1717-1722
[26] P Loschialpo, C A Kapetanakos. High-current density, high-brightness electron beams from large-area lanthanum hexaboride cathodes. Journal of Applied Physics, 1988, 63(8): 2552-2557
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