The full densification polycrystalline cerium hexaboride （CeB6） cathode material was prepared by using the spark plasma sintering （SPS） method in an oxygen free system. The starting precursor nanopowders with an a...The full densification polycrystalline cerium hexaboride （CeB6） cathode material was prepared by using the spark plasma sintering （SPS） method in an oxygen free system. The starting precursor nanopowders with an average grain size of 50 nm were prepared by high-energy ball milling. The ball-milled nanopowder was fully densified at 1550 °C under 50 MPa, which was about 350 °C lower than the conventional hot-pressing method and it was also lower than that of coarse powder under the same sintering condition. The mechanical properties of nanopowder sintered samples were significantly better than that of coarse powder, e.g., the flexural strength and Vickers hardness were 211% and 51% higher than that of coarse powder, respectively. The electron backscattered diffraction （EBSD） result showed that the （100） fiber texture could be fabricated by the ball-milled nanopowder sintered at 1550 °C and the thermionic emission current density was measured to be 16.04 A/cm2 at a cathode temperature of 1873 K.展开更多
Large-sized and high-quality cerium hexaboride(CeB6)single crystals are successfully grown by the optical floating zone method.The structure,chemical composition and thermionic emission properties of the crystal are c...Large-sized and high-quality cerium hexaboride(CeB6)single crystals are successfully grown by the optical floating zone method.The structure,chemical composition and thermionic emission properties of the crystal are characterized by x−ray diffraction,x-ray fluorescence and emission measurements,respectively.Based on the observation of single crystal diffraction,the relative density of feed rods has a great effect on the quality of the grown crystal.The thermionic emission measurement results show that the emission current density of the single crystal is 47.1 A/cm^(2) at 1873 K with an applied voltage of 1 kV,which is about two times larger than the value for polycrystalline samples.The single crystal possesses excellent emission current stability.Therefore,it is expected that CeB6 single crystal is a very promising material for thermionic cathode applications.展开更多
The grain size controlled bulk Lanthanum hexaboride （LAB6） cathode material was prepared by using the spark plasma sintering method in an oxygen free system. The starting precursor nanopowders with average grain siz...The grain size controlled bulk Lanthanum hexaboride （LAB6） cathode material was prepared by using the spark plasma sintering method in an oxygen free system. The starting precursor nanopowders with average grain size of 50 nm were prepared by high-energy ball milling. The nanopowder was fully densified at 1300℃, which is about 350℃ lower than the sintering temperature of the coarse powders. The thermionic emission current density was measured to be 42.0 A/cm2, which is much higher than 24.2 A/cm2 of coarse powders and Vickers hardness to be 1860 kg/mm2, which is also higher than 1700 kg/mm2 of coarse one. These results indicate that refining the powder grain size to nano level was beneficial for reducing the sintering temperature and promoting the thermionic emission and mechanic properties.展开更多
The linear Langmuir and electromagnetic (EM) waves in relativistic hot plasmas are discussed, and the dispersion relations are obtained based on the covariant Maxwell's and fluid equations. When k_BT/mc^2> 1, t...The linear Langmuir and electromagnetic (EM) waves in relativistic hot plasmas are discussed, and the dispersion relations are obtained based on the covariant Maxwell's and fluid equations. When k_BT/mc^2> 1, the effective mass of electrons will be increased obviously. As the results, many other influences are induced, such as the decrease of the plasmas frequency and the critical frequency, the reduction of the electron sound velocity and the electrons' oscillation velocity, and so on. Numerical results show that these influences can affect the dispersion relations of Langmuir and EM waves seriously even in linear regime.展开更多
Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on...Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.展开更多
基金Project supported by the National Natural Science Foundation of China (50871002)
文摘The full densification polycrystalline cerium hexaboride （CeB6） cathode material was prepared by using the spark plasma sintering （SPS） method in an oxygen free system. The starting precursor nanopowders with an average grain size of 50 nm were prepared by high-energy ball milling. The ball-milled nanopowder was fully densified at 1550 °C under 50 MPa, which was about 350 °C lower than the conventional hot-pressing method and it was also lower than that of coarse powder under the same sintering condition. The mechanical properties of nanopowder sintered samples were significantly better than that of coarse powder, e.g., the flexural strength and Vickers hardness were 211% and 51% higher than that of coarse powder, respectively. The electron backscattered diffraction （EBSD） result showed that the （100） fiber texture could be fabricated by the ball-milled nanopowder sintered at 1550 °C and the thermionic emission current density was measured to be 16.04 A/cm2 at a cathode temperature of 1873 K.
基金by the National Natural Science Foundation of China under Grant No 50871002.
文摘Large-sized and high-quality cerium hexaboride(CeB6)single crystals are successfully grown by the optical floating zone method.The structure,chemical composition and thermionic emission properties of the crystal are characterized by x−ray diffraction,x-ray fluorescence and emission measurements,respectively.Based on the observation of single crystal diffraction,the relative density of feed rods has a great effect on the quality of the grown crystal.The thermionic emission measurement results show that the emission current density of the single crystal is 47.1 A/cm^(2) at 1873 K with an applied voltage of 1 kV,which is about two times larger than the value for polycrystalline samples.The single crystal possesses excellent emission current stability.Therefore,it is expected that CeB6 single crystal is a very promising material for thermionic cathode applications.
基金Supported by the National Natural Science Foundation of China under Grant No 50871002.
文摘The grain size controlled bulk Lanthanum hexaboride （LAB6） cathode material was prepared by using the spark plasma sintering method in an oxygen free system. The starting precursor nanopowders with average grain size of 50 nm were prepared by high-energy ball milling. The nanopowder was fully densified at 1300℃, which is about 350℃ lower than the sintering temperature of the coarse powders. The thermionic emission current density was measured to be 42.0 A/cm2, which is much higher than 24.2 A/cm2 of coarse powders and Vickers hardness to be 1860 kg/mm2, which is also higher than 1700 kg/mm2 of coarse one. These results indicate that refining the powder grain size to nano level was beneficial for reducing the sintering temperature and promoting the thermionic emission and mechanic properties.
基金Supported by the National Natural Science Foundation of China under Grant Nos.11064005 and 51460020the financial support provided by the Key Subject of Atomic and Molecular Physics in Jiangxi Province(2011-1015)
文摘The linear Langmuir and electromagnetic (EM) waves in relativistic hot plasmas are discussed, and the dispersion relations are obtained based on the covariant Maxwell's and fluid equations. When k_BT/mc^2> 1, the effective mass of electrons will be increased obviously. As the results, many other influences are induced, such as the decrease of the plasmas frequency and the critical frequency, the reduction of the electron sound velocity and the electrons' oscillation velocity, and so on. Numerical results show that these influences can affect the dispersion relations of Langmuir and EM waves seriously even in linear regime.
基金Project supported by the National Natural Science Foundation of China (Grant No. 50871002)
文摘Polycrystalline rare-earth hexaborides （NdxGdl-x）B6 （x = 0, 0.2, 0.6, 0.8, 1） were prepared by the reactive spark plasma sintering （SPS） method using mixed powder of GdH2, NdH2 and B. The effects of Nd doping on the crystal structure, the grain orientation, the thermionie emission and the magnetic properties of the hexaboride were investigated by X-ray diffraction, electron backscattered diffraction and magnetic measurements. It is found that all the samples sintered by the SPS method exhibit high densities （〉95%） and high values of Vickers hardness （2319 kg/mm2）. The values are much higher than those obtained in the traditional method. With the increase of Nd content,the thermionic emission current density increases from 11 to 16.30 A/cm2 and the magnetic phase transition temperature increases from 5.85 to 7.95 K. Thus, the SPS technique is a suitable method to synthesize the dense rare-earth hexaborides with excellent properties.
