Thermionic Emission Properties Research Articles

Improvement of the thermionic emission properties of C12A7 electride

Calcium aluminate electride (named as C12A7) has become an active topic as a material for electron emission in the research field of electron sources, especially for electric propulsion, in recent years. Whereas the initially predicted, extremely low work function of 0.6 eV could not be confirmed experimentally at technically relevant surface current densities, the material remains an attractive emitter. Besides C12A7 emitting electrons at lower temperatures compared to LaB6, it appears to be resistant to poisoning and even iodine compatible as only known low work function material. On the other hand, the ceramic electride nature of the emitter introduces challenges related to the thermal, mechanical and electrical properties. Therefore, C12A7 has been investigated experimentally as a candidate material for thermionic emitters in space applications. The aim of this study was to enhance the thermionic emission by material modifications such as surface treatments, electrical contacting methods and doping of the C12A7. The performance improvement was quantified in a thermionic diode configuration, in which the emission current was determined as a function of temperature over multiple heating cycles to observe time dependent effects. The surface treatment such as grinding or polishing affects the emission properties of the C12A7 negatively. In contrary, the doping of the C12A7 lattice as well as the contacting of the back side (heater side) significantly improved the emission as well as the measured current density.

Ultrahigh-gain organic transistors based on van der Waals metal-barrier interlayer-semiconductor junction.

Intrinsic gain is a vital figure of merit in transistors, closely related to signal amplification, operation voltage, power consumption, and circuit simplification. However, organic thin-film transistors (OTFTs) targeted at high gain have suffered from challenges such as narrow subthreshold operating voltage, low-quality interface, and uncontrollable barrier. Here, we report a van der Waals metal-barrier interlayer-semiconductor junction-based OTFT, which shows ultrahigh performance including ultrahigh gain of ~104, low saturation voltage, negligible hysteresis, and good stability. The high-quality van der Waals-contacted junctions are mainly attributed to patterning EGaIn liquid metal electrodes by low-energy microfluidic processes. The wide-bandgap semiconductor Ga2O3 as barrier interlayer is achieved by in situ surface oxidation of EGaIn electrodes, allowing for an adjustable barrier height and expected thermionic emission properties. The organic inverters with a high gain of 5130 and a simplified current stabilizer are further demonstrated, paving a way for high-gain and low-power organic electronics.

Hollow cathode testing of Y2O3, La2O3-doped tungsten and LaB6 emitters with krypton and iodine

In iodine electric propulsion the remaining challenge is an iodine fed hollow cathode to enable single propellant subsystems for higher power levels. In this pursuit novel hollow cathode emitter materials such as Y2O3 and La2O3-doped tungsten have been tested both with krypton and iodine and are compared to LaB6 as conventional emitter. LaB6 and Y2O3 failed to ignite with iodine, whereas the La2O3-doped tungsten degraded to be unusable within only 2.4 h through erosion and depletion. For krypton operation, the doped tungsten performed similar to LaB6 requiring keeper currents of 1.2 A to extract 0.3 A at the anode in spot-mode at flow rates below 1 sccm. The Y2O3 showed promising performance, allowing lower flow rates and power consumption. Degradation occurred however after 600 h testing and the emitter was destroyed by subsequent exposure to air. To characterize the materials, the thermionic emission properties of the three emitters have been determined by measuring current-temperature relations. Although the two evaluated emitter materials did operate satisfactory on krypton, they show only limited lifetime and performance compared to conventional emitters and are not iodine compatible.

Effects of Sm doping on EuB6

A solid-state reaction was used to investigate the nanocrystalline particles of Sm-doped EuB6 and their optical, thermionic emission and mechanical properties were investigated. The tapered nanoawls had a length of 3–12 μm and a diameter ranging from 40 to 200 nm at the roots and 20–100 nm at the tip as shown by in scanning electron microscopy (SEM). As the temperature of the material increases, the thermionic emission current density also increases. Jo as the zero-field current densities for Eu0.6Sm0.4B6 at 1500 K, 1673 K, 1773 K, 1873 K were 0.72 A cm-2, 4.25 A cm-2, 10.06 A cm-2 and 20.05 A cm-2. By increasing the Sm doping content, electrical density of Eu1-xSmxB6 decreases. In all materials, the electrical resistivities increased linearly with temperature from 200 to 1200 °C, indicating metallic conductivity. Eu0.6Sm0.4B6 has a lower Vickers hardness and higher flexural strength than EuB6.

Optical floating zone growth and thermionic emission properties of La0.9Sr0.1B6 single crystal

This paper has studied the crystal structure, growth orientation, crystal quality and thermionic emission properties of the La0.9Sr0.1B6 single crystal prepared by the optical floating zone method. By the means of XRD, single crystal X-ray diffraction, X-ray Laue diffraction, X-ray rocking curve and TEM, the La0.9Sr0.1B6 single crystal grown along [100] orientation is determined to be a single-phase solid solution of Sr-doped LaB6 with the typical cubic structure and have a good crystal quality and integrity. The higher J1kV (42.55 A/cm2), Jo (31.19 A/cm2) and lower Φeff (2.492 eV) than LaB6 under the cathode operating temperature of 1500 °C as well as the good thermionic emission stability strongly suggest that the grown La0.9Sr0.1B6 single crystal has a good potential to be a promising hot cathode material.

The densification behavior, microstructure, mechanical, and thermionic emission properties of LaB6‐SiC composite

AbstractThe LaB6‐SiC composite with the different SiC content (0, 15, 30, 36, 50, 90, and 100 wt.%,) was densified by spark plasma sintering. The effects of SiC content on the densification behavior, microstructure, mechanical, and thermionic emission properties of LaB6‐SiC composite were systemically investigated. The results show that all the rapid shrinkage occurred at the heating stage during densification, and LaB6‐36 wt.%SiC composite owned the maximum shrinkage rate of 1.5 mm/min at T = 1798°C. The highest relative density of the composite decreased from 98.18% to 95.01% as the SiC content increased from 15 wt.% to 90 wt.%, under which the morphology of LaB6 grain evaluated from the equiaxed to elongated structure, and LaB6 grain size varied in the range of 5.05–11.42 μm. The similar eutectic structures were observed in the LaB6‐36 wt.% SiC composite because of some LaB6 grains melting. Both the highest fracture toughness of 5.15 ± 0.56 MPa.m1/2 and the highest bending strength of 313 ± 4.7 MPa belonged to the LaB6‐36 wt.% SiC composite, which also exhibited thermionic emission current density of 10.74 A/cm2 and work function of 2.99 eV at T = 1873 K.

Spark plasma sintering and characterization of bulk La1-xSrxB6 hexaborides

Herein, bulk La1-xSrxB6 hexaborides with >96.5% in relative densities have been fabricated by SPS, and the phase composition, microstructure, mechanical and thermionic emission properties are investigated. The sintered specimens with average grain sizes of about 14 μm are determined to be CsCl-type single-phase solid solutions without any detectable impurities or other secondary phases. The homogenous elemental distributions of La and Sr have been confirmed at both microscale and nanoscale. The Vickers hardness (18–23 GPa) of these sintered specimens are higher than LaB6, while the indentation fracture toughness (about 2.00 MPa m1/2) are comparable to LaB6. The nanoindentation hardness and Young's moduli have been measured to be 26–30 GPa and 380–400 GPa, respectively, by nanoindentation tests at a maximum load of 20 mN. Only from the perspective of work function, bulk La1-xSrxB6 hexaborides with lower work functions and higher thermionic emission current densities than LaB6 have a potential to be a class of excellent hot cathodes.

Experimental investigation of thermionic cathode properties of ultrafine tungsten filament

The thermionic cathode emission properties of an ultrafine tungsten filament are investigated with the accurate filament temperature (T) measured by an infrared pyrometer. The results show that the tungsten filament begins to emit electrons at T of about 1800 K, and the maximum electron emission efficiency (∼20 mA/W) occurs at T of about 2900 K. Moreover, the effective filament area for electron emission reaches saturation after T exceeds 2700 K. For a tungsten hot-cathode, a using standard is recommended that its operating temperature should not exceed 2600 K, the electron emission current density should not exceed 0.5 A/cm2, and the electron emission efficiency should be less than 5 mA/W. Additionally, the relation between the thermal emissivity of tungsten filament and T is also studied, showing a nearly linear increase of the thermal emissivity from 0.23 to 0.31 with T increasing from 1700 K to 2500 K and a saturated thermal emissivity of 0.48 at T of about 2900 K.

Single-step low-temperature synthesis routes for (Ba, Ce, Gd) hexaborides and their thermionic emission properties

Rare-earth/alkaline-earth hexaborides with high chemical stability and low work function have recently attracted much interest as promising thermionic cathodes. Their synthesis at low temperatures without post-synthesis purification treatments is still a significant challenge. The present work demonstrates a simple synthesis technique to produce pure barium, cerium and gadolinium hexaborides (BaB6, CeB6, and GdB6). For each hexaboride, a batch of samples was created by first milling highly pure precursor powders and then firing at various temperatures in a low-vacuum furnace. The synthesised products were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron dispersive spectroscopy (EDS). Their thermionic properties were investigated utilising a Schottky device. Pure BaB6, CeB6, and GdB6 were successfully synthesised at lower temperatures (≤1400 °C). Nanocrystalline CeB6 was found to be produced at the lowest temperature of 1200 °C. The mean particle size of pure CeB6 and GdB6 synthesised is seen to be in the sub-micron range, with the exception of large BaB6 particles. The estimated work functions of the BaB6, CeB6, and GdB6 cathodes are 1.61 ± 0.18 eV, 2.50 ± 0.09 eV, and 2.42 ± 0.09 eV, respectively. These findings imply that hexaborides, as promising thermionic emitters, can be easily produced at low temperatures.

Electronic structures and emission properties of typical binary single crystal REB<sub>6</sub>

Binary rare earth hexaborides (REB<sub>6</sub>) have different rare earth elements with different valence electron distributions, which lead to different strange physical properties and different emission properties. However, in the electron emission properties, whether PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub> and GdB<sub>6</sub> all have excellent emission properties remains to be further studied, and the physical mechanism affecting their emission properties needs investigating. In this paper, the electronic structures, work functions of typical binary single crystal REB<sub>6</sub> (LaB<sub>6</sub>, CeB<sub>6</sub>, PrB<sub>6</sub>, NdB<sub>6</sub>, SmB<sub>6</sub>, GdB<sub>6</sub>) are studied by first principles calculations. The single crystal REB<sub>6</sub> are prepared by optical zone melting method, and their thermionic electron emission properties are tested experimentally. The theoretical calculation results show that the typical binary REB<sub>6</sub> have large densities of states near the Fermi level. The d-orbitals with broad distributions in conduction bands are beneficial to electron emission. The localized f-orbital electrons in valence bands are not conducive to their electron emission. The theoretical calculations of work functions of typical binary single crystal REB<sub>6</sub> (100) surface are consistent with the analyses of their electronic structures. The theoretical calculation values of work functions are ordered as GdB<sub>6</sub> (2.27 eV) < CeB<sub>6</sub> (2.36 eV) < LaB<sub>6</sub> (2.40 eV) < PrB<sub>6</sub> (2.58 eV) < SmB<sub>6</sub> (2.63 eV) < NdB<sub>6</sub> (2.91 eV). The experimental test results of thermionic electron emission of single crystal show that the experimental thermionic electron properties are consistent with the theoretical ones. The LaB<sub>6</sub> and CeB<sub>6</sub> both have good thermionic and field emission properties, and the GdB<sub>6</sub> has excellent field emission properties.

Structure and properties of La0.6Ce0.3Pr0.1B6‐ZrB2, a directionally solidified eutectic with different growth rates

AbstractLa0.6Ce0.3Pr0.1B6‐ZrB2, a directionally solidified eutectic (DSE), is a material with a high‐performance thermionic emission with great application prospects. The influence of the growth rates of this DSE on the microstructure, growth direction, two phases orientation relationship, mechanical properties, and thermionic emission properties was investigated. The experimental results show that the microstructure and growth direction of the two phases varied with the growth rate of the eutectic: the faster the growth rate, the lower the uniformity of the ZrB2 phase became, and the growth directions of the La0.6Ce0.3Pr0.1B6 matrix and the ZrB2 phase were also altered accordingly. In the La0.6Ce0.3Pr0.1B6‐ZrB2 DSE with a growth rate of 100 mm/h, the two phases have a unique growth direction and a crystallographic orientation relationship. The fracture toughness of the La0.6Ce0.3Pr0.1B6‐ZrB2 DSE is at least 50% higher than that of the single crystal La0.6Ce0.3Pr0.1B6, and the fracture toughness of the DSEs formed at different growth rates also differed.

Thermionic emission and conversion properties of n-type AlGaN thin film cathodes grown on 6H–SiC substrates

We observed the thermionic emission (TE) and conversion (TC) characteristics of Si-doped AlGaN thin film cathodes grown on n-type 6H-SiC substrates with Cs adsorption on the surfaces. The threshold temperatures of TE decreased as the AlN mole fraction was increased, with threshold temperatures of around 205 °C for Si–doped Al0.5Ga0.5N. TE was detected even when the applied external voltage was 0 V at 600 °C, which means that the cathodes had TC characteristics at a temperature dramatically lower than that of conventional systems with metal-based cathodes. In addition, we observed a notable enhancement of the emission current when the cathodes were irradiated with white light having energy lower than that of the bandgap of the AlGaN films at low temperatures. We suggest that the enhancement was caused by an internal photoelectron effect due to the absorption of the light in the n–type 6H–SiC substrate.

Theory of Thermionic and Secondary Emission Properties of Palladium–Barium Cathodes of Microwave Electrovacuum Devices

A model of the surface structure of a palladium–barium cathode is proposed and the Dionne secondary electron emission model is used to calculate the dependence of the secondary electron emission coefficient of barium oxide crystallites forming on the surface of a cathode during its thermal activation on the temperature and oxygen vacancy concentration in the oxide. It is shown that the emission properties of a palladium–barium cathode significantly depend on both the emission properties of barium oxide crystallites and the cathode relative surface area occupied by them. The theoretical nomograms are built that relate the measured surface-averaged work function to the secondary electron emission coefficient of a palladium–barium cathode; the nomogram parameters are the temperature, oxygen vacancy concentration in barium oxide crystallites, and the relative cathode surface area occupied by them.

Preparation, characterization and properties of La0.6Ce0.3Pr0.1B6-ZrB2 directionally solidified eutectic grown via the optical floating zone technique

La0.6Ce0.3Pr0.1B6-ZrB2 directionally solidified eutectic (DSE) with the [100] orientation of the La0.6Ce0.3Pr0.1B6 matrix is firstly fabricated via the optical floating zone technique using LaB6, CeB6, PrB6 and ZrB2 powders as raw materials. The microstructure, mechanical and thermionic emission properties of the as-grown La0.6Ce0.3Pr0.1B6-ZrB2 DSE were systematically investigated. The ZrB2 fibers, whose average diameter is 0.8–1.2 μm, are embedded in the [100] La0.6Ce0.3Pr0.1B6 matrix and uniformly distributed. The Vickers hardness does not vary apparently between the transverse and longitudinal sections of La0.6Ce0.3Pr0.1B6-ZrB2, while the fracture toughness is of highly anisotropy. The fracture toughness of 5.71 MPa m1/2 on the longitudinal section, flexural strength of 826.50 MPa and thermionic emission current density of 84.96 A/cm2, which are all higher than that of LaB6 or LaB6-ZrB2, indicate that the La0.6Ce0.3Pr0.1B6-ZrB2 DSE grown here will have great application prospects as a candidate for high-performance thermionic emitters.

Crystal growth and thermionic emission properties of Ce1-x-yLaxPryB6 single crystals

CeB6-based quaternary rare-earth hexaboride single crystals Ce1-x-yLaxPryB6 (x = 0.1–0.4, y = 0.1–0.4) with Φ 5 mm × 35 mm were grown via the optical floating zone technique. The crystal structure, atomic composition and thermionic emission properties of Ce1-x-yLaxPryB6 single crystals were systematically studied. The results of XRD and X-ray Laue diffraction reveal that these single crystals are grown along the <100> direction. The X-ray rocking curve analysis shows that the lowest full-width at half-maximum (FWHM) of Ce1-x-yLaxPryB6 single crystals is only 0.036° for Ce0.4La0.4Pr0.2B6 single crystal, whose atomic structure is revealed and the lattice spacing is 0.414 nm. The maximum thermionic emission current density of 114.21 A/cm2 at 1600 °C and minimum effective work function of 2.62 eV at the same temperature are also belonged to Ce0.4La0.4Pr0.2B6 single crystal, which indicates that it has great application prospects as a promising hot cathode material.

Fabrication, microstructure, and enhanced thermionic electron emission properties of vertically aligned nitrogen-doped nanocrystalline diamond nanorods

Abstract

High-quality LaxCeyPr1-x-yB6 single crystal with excellent thermionic emission properties grown by optical floating zone melting method

The high-quality LaxCeyPr1-x-yB6 (x = 0.6–0.8, y = 0.1–0.3) single crystals have been firstly grown by optical floating zone melting method. The crystal structure, mechanical and thermionic emission properties of the as-grown single crystals were characterized by X-ray single crystal diffractometer, SEM, XRD, X-ray Laue diffraction. Subgrain-boundary-free [100] LaxCeyPr1-x-yB6 single crystal with a good crystalline integrity and homogeneous element distribution is obtained. The Lattice constant of LaxCeyPr1-x-yB6 single crystal is lower than that of the LaB6 single crystal. The lowest FWHM of 162.0 arcsec belongs to the La0.6Ce0.3Pr0.1B6 single crystal. The highest thermionic emission current density of 105.10 A/cm2 and the lowest work function of 2.562 eV indicate that La0.6Ce0.3Pr0.1B6 single crystal is a very promising thermionic cathode material.

The thermionic and field emission properties of single crystal PrB6 grown by floating zone method

The high-quality Praseodymium hexaboride (PrB6) single crystal was successfully grown by a floating zone method using PrB6 powders as starting materials. The XRD pattern and single crystal diffractometer confirmed that the high-quality PrB6 single crystal could be successfully obtained. The theoretical calculation results show that the d orbital electrons of Pr atoms contribute the electron emission of PrB6. Thermionic emission test results show that the work function of single crystal PrB6 (100) surface is 2.73 eV. The field emission study shows that a field emission current of 8.2 μA was drawn at the electric field of 8.8 V/μm from the single tip corroded by PrB6 single crystal. Thus, the PrB6 single crystals could have potential application prospect as a thermionic and field emission cathode material.

Surface electronic structures and emission property of single crystal LaB6 typical surfaces

The electron emission properties of lanthanum hexaboride (LaB6) have received much attention because its low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, single crystal LaB6 is an ideal thermionic emission and field emission cathode material, its different crystal surfaces exhibit different emission properties. So far the physical factors of the emission properties of different crystal surfaces of LaB6 single crystal have been rarely reported. In this paper, the density function theory based first-principles calculations are used to analyze the electron density differences, band structures and densities of states of the typical LaB6 (100), (110), (111), (210), (211) and (310) surfaces, and the thermionic emission properties of the high-quality single crystal LaB6 typical surfaces are tested. The theoretical calculation results show that single crystal LaB6 has metal properties, electron emission characteristics and anisotropy of emission performance which are mainly caused by different crystal structures and electronic structures of LaB6 typical surfaces. The densities of La atoms in different surfaces of LaB6 single crystal are different, and a high density of La atoms in a surface is beneficial to its emission performance. The difference between relative positions for the Fermi level of different surfaces has different effect on their emission performance, and a surface with high position of Fermi level against the bottom of conduction band could have small work function and good emission performance. In addition, a surface structure of single crystal LaB6 has a large density of states and a high number of distributions of conduction band near the Fermi level, which are in favor of its electron emission. The (100) surface of single crystal LaB6 with the highest density of La atoms and electronic structure in favor of electron emission could have optimal electron emission performance compared with the remaining crystal surfaces. Thermionic emission test results show that maximum emission current densities of the (100), (110), (111), (210), (211) and (310) surfaces are 42.4, 36.4, 18.4, 32.5, 30.5 and 32.2 A/cm2 at the cathode temperature 1773 K and the voltage 1 kV. The (100) surface of LaB6 single crystal has a maximum emission current density under the same test condition, meaning that this surface has a smallest work function and best emission property compared with the other crystal surface. The thermionic emission test results show that the actual performances are basically accordant with the calculated results, demonstrating that the first principle calculation could provide a good theoretical guidance for studying the electron emission properties of rare earth hexaborides (REB6) and other cathode materials.

Preparation and Properties of (Ce0.4La0.2Pr0.2Nd0.2)B6 Single‐Crystal by the Optical Float‐Zone Traveling Solvent Technique

AbstractThe high quality (Ce0.4La0.2Pr0.2Nd0.2)B6 single‐crystal has been successfully prepared by the optical float‐zone traveling solvent technique. The crystal structure and performance of the (Ce0.4La0.2Pr0.2Nd0.2)B6 single‐crystal obtained at different rates are characterized by using XRD, X‐ray single‐crystal diffraction, rotating target X‐ray Diffractometer, X‐ray Laue diffraction, SEM and thermionic emission equipment etc.. The results show that the as‐grown single‐crystal with the uniform composition has a cubic structure with the lattice size of 4.143 Å. The optimal mechanical and thermionic emission properties belong to the (Ce0.4La0.2Pr0.2Nd0.2)B6 single‐crystal prepared at growth rate V = 7 mm/h. The values of 53.3 A/cm2 in the current density at 1873 K under the applied voltage of 4 kV and 29.26 GPa in Vickers hardness indicate that (Ce0.4La0.2Pr0.2Nd0.2)B6 single‐crystal is a promisingly thermionic emission material.