Thermionic Emission Measurements Research Articles

Thermionic emission measurement of sintered lanthanum hexaboride discs and modelling of their solar energy conversion performance

Polycrystalline lanthanum hexaboride (LaB6) discs were sintered by hot pressing of commercial powders and successively polished, in order to investigate their thermionic emission capability. LaB6 samples presented values of work function in the range 2.55–2.61 eV. Discs with polished surface, independently from the composition and density, demonstrated to provide a Richardson constant around 45 A cm−2 K−2. These results are promising for the use of sintered LaB6 discs in thermionic energy converters operating at temperatures in the 1400–1700 °C range. Moreover, since the properties of solar absorption and spectral selectivity of LaB6 make them appealing for the conversion of concentrated solar radiation, a modelling study to estimate the energy conversion efficiency was performed, aimed at identifying the proper engineering strategies able to make the resulting devices competitive for electrical conversion of concentrated solar radiation.

Nanocrystalline lanthanum boride thin films by femtosecond pulsed laser deposition as efficient emitters in hybrid thermionic-photovoltaic energy converters

Lanthanum hexaboride (LaB6)-based thin films were successfully synthesized via femtosecond Pulsed Laser Deposition (fs-PLD) at room temperature and at high growth rate (>110 nm/min) for acting as electron and photon emitters in hybrid thermionic-photovoltaic devices applied in thermal-to-electrical energy conversion. The physical and chemical properties of the films were systematically investigated as a function of the most relevant deposition parameters (laser pulse repetition rate, pressure of the reactor, and deposition time) aimed at optimizing the material’s microstructure and functional optical and electronic properties. Specific growth conditions allowed for the achievement of thin films consisting of nanocrystalline LaB6 (10–20 nm grain-size) with enhanced spectral emissivity and low work function of (2.64 ± 0.03) eV, evaluated by thermionic emission measurements up to 1860 K, where a thermionic current density of 1.78 A cm−2 was measured. The obtained results demonstrate that fs-PLD represents a novel and rapid method for preparing efficient, and low-cost thermal electron emitters for innovative hybrid thermionic conversion devices operating at temperatures close to 2000 K.

Structural, magnetic, and thermionic emission properties of multi-functional La1-xCaxB6 hexaboride

Herein, we report the synthesis of nanocrystalline La1-xCaxB6 (0 ≤ x ≤ 1) hexaboride powders by solid-state reaction and their subsequent consolidation via spark plasma sintering. The structural, magnetic and thermionic emission properties of La1-xCaxB6 hexaboride are investigated. All of the synthesized nanocrystalline hexaboride powders are single phase with the CsCl-type structure and no ferromagnetic impurity phases have been detected from X-ray diffraction. Magnetic measurements show that weak ferromagnetism at room temperature is found in nanocrystalline La1-xCaxB6 hexaboride powders, and the magnetism was attributed to the presence of the intrinsic defects, based on the data of the HRTEM. Thermionic emission measurements indicate that the maximum emission intensity for bulk La0.4Ca0.6B6 at 1873 K reached 20.02 A/cm2, which is more than three times higher as compared to bulk CaB6 (∼6.04 A/cm2). When the La doping was increased to 40 at%, the work function of CaB6 decreased from 2.95 to 2.76 eV, indicating an improvement in the thermionic emission performance. Therefore, the quasibinary La1-xCaxB6 hexaboride may have an application as a promising cathode.

Synthesis and characterization of low work function alkali oxide thin films for unconventional thermionic energy converters

In this work, we present the synthesis and the characterization of low work function thin films for Micro Thermionic Converters (MTC). The objective is producing a device operating at relatively low temperature (<1000 K). We aim at improving the MTC efficiency by reducing the work function of the electrodes and increasing the emitted current density by alkali metal oxides electrodes coating. In particular, in this work, we analyse and compare the performances of two alkali metal oxides: potassium and caesium oxides. Our choice to exploit those materials relies on their low work function and their abundance. For both materials, we present the results on the synthesis of the oxides under high vacuum and controlled temperature. The oxide thin films were characterized by X-ray photoelectron spectroscopy, photoemission, and thermionic emission measurements. By exploiting the latter technique, a quantitative evaluation of the current density, emitted by the heated oxides, is obtained as a function of temperature. Our results demonstrate that it is possible to decrease the silicon work function by almost 3 eV, enabling significant thermionic currents despite relatively low temperatures (below 850 K).

Characteristics of LaB6 thin films grown by pulsed laser deposition

LaB6 thin films were deposited at a temperature of 500 °C under vacuum or Ar atmosphere by the pulsed laser deposition technique on (100) Si substrates using a KrF laser. Grazing incidence x-ray diffraction investigations found that films were nanocrystalline, with grain size dimensions from 86 to 102 nm and exhibited microstrain values around 1.1%. Simulations of the x-ray reflectivity curves acquired from the deposited films showed that films had a density around 4.55 g/cm3, and were very smooth, with a surface roughness root-mean-square of 1.5 nm, which was also confirmed by scanning electron and atomic force microscopy measurements. All films were covered by a ∼2 nm thick contamination layer that formed when samples were exposed to the ambient. Auger electron spectroscopy investigations found very low oxygen impurity levels below 1.5 at. % once the contamination surface layer was removed by Ar ion sputtering. Four point probe measurements showed that films were conductive, with a resistivity value around 200 μΩ cm for those deposited under Ar atmosphere and slightly higher for those deposited under vacuum. Nanoindentation and scratch investigations showed that films were rather hard, H ∼ 16 GPa, E ∼ 165 GPa, and adherent to the substrate. Thermionic emission measurements indicated a work function value of 2.66 eV, very similar to other reported values for LaB6.

Thermionic emission from phosphorus (P) doped diamond nanocrystals supported by conical carbon nanotubes and ultraviolet photoelectron spectroscopy study of P-doped diamond films

Materials with low work function values (<2eV) are highly in demand for low temperature thermionic electron emission, which is a key phenomenon for waste heat recovery applications. Here we present the work function reduction of phosphorus (P) doped (i) diamond nanocrystals grown on conical carbon nanotubes (CCNTs) and (ii) diamond films grown on silicon substrates. Thermionic emission measurements from phosphorus doped diamond crystals on CCNTs resulted in a work function value of 2.23eV. The CCNTs provide the conducting backbone for the P-doped diamond nanocrystals and the reduced work-function is interpreted as due to the presence of midband-gap state and no evidence for negative electron affinity was seen. However, ultraviolet photoelectron spectroscopy studies on phosphorus doped diamond films yielded a work function value of ~1.8eV with a negative electron affinity (NEA) value of 1.2eV. Detailed band diagrams are presented to support the observed values for both cases.

Large emission current density of LaxCe1−xB6 high quality single crystals grown by floating zone technique

The high quality LaxCe1−xB6 (x=0, 0.2, 0.4, 0.6, 1) single crystals with large emission current density have been successfully grown by floating zone method. The crystal structure, crystal quality and emission properties of LaxCe1−xB6 single crystals were characterized by 360° Phi scanning single crystal diffractometer, X-ray Laue diffraction and the thermal electron emission measurements. It is found that the as-grown crystals have a good crystal quality without subgrains and twin crystal phenomenon. The thermionic emission measurement results show that the maximum emission current density on (100) surface of La0.2Ce0.8B6 single crystal is 71.2A/cm2 at 1873K under an applied voltage of 800V, which is larger than the reported values in the literatures up to now. Furthermore, the crystals with different components also show a higher emission current density such as 53.8A/cm2 for La0.4Ce0.6B6 and 53.3A/cm2 for CeB6 single crystals. The field emission single tip made by LaB6 single crystal shows a superior performance than the polycrystalline LaB6 field emission tip and its field enhancement factor (β) is 36,231cm−1. Thus, the LaxCe1−xB6 single crystals will have promising application prospects as a hot or cold cathode.

Floating Zone Growth and Thermionic Emission Property of Single Crystal CeB6

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/cm2 at 1873 K with an applied voltage of 1 kV, which is about two times larger than the value for poly crystalline 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.

Thermionic electron emission from nitrogen-doped homoepitaxial diamond

Nitrogen-doped homoepitaxial diamond films were synthesized for application as low-temperature thermionic electron emitters. Thermionic electron emission measurements were conducted where the emission current was recorded as a function of emitter temperature. At a temperature < 600 °C an emission current was detected which increased with temperature, and the emission current density was about 1.2 mA/cm 2 at 740 °C. The electron emission was imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM). The image displayed uniform electron emission over the whole surface area. Thermionic emission and ultraviolet photoemission spectroscopy were employed to determine the temperature dependent electron emission energy distribution from the nitrogen-doped homoepitaxial diamond films. The photoemission spectra indicated an effective work function of 2.4 eV at 550 °C. These values indicate reduced band bending and establish the potential for efficient electron emission devices based on nitrogen-doped homoepitaxial diamond.

Thermionic electron emission from low work-function phosphorus doped diamond films

Thermionic electron emitters are a key component in applications ranging from travelling wave tubes for communications, space propulsion and direct energy conversion. As the conventional approach based on metallic emitters requires high operating temperatures the negative electron affinity (NEA) characteristic of diamond surfaces in conjunction with suitable donors would allow an electronic structure corresponding to a low effective work function. We have thus prepared phosphorus-doped polycrystalline diamond films on metallic substrates by plasma assisted chemical vapor deposition where an NEA surface characteristics was induced by exposure of the film surface to a hydrogen plasma. Thermionic electron emission measurements in an UHV environment were conducted with respect to the Richardson–Dushman relation observing an emission current at temperatures < 375 °C. Measurements were terminated at 765 °C without significant reduction in the electron emission current indicating a stable hydrogen passivation of the diamond surface. A fit of the emission data to the Richardson equation allowed for the extraction of emission parameters where the value of the materials work function was evaluated to 0.9 eV. This value could well be the lowest measured work function of any known material.

Measurement of Work Function of Practical Fluorescent Lamp Electrodes with Emissive Oxides

In this paper we propose a method for measuring work function of practical fluorescent lamp electrodes with emissive oxides. The work function measurement is based on the matching between the zero-field thermionic emission currents and various dc lamp currents set in a normal glow discharge by means of a lamp voltage monitoring procedure with an electrode-heating device for thermionic emission measurement. Measuring the work function of practical fluorescent lamp electrodes is a promising approach because changes that occur in the work function of aged-lamp electrodes in some lighting systems will be helpful in evaluating the system. The paper points out the doubtful approach of taking a rather small value of the Richardson constant. To solve this issue, a temperature-dependent work function is introduced. In the first stage of the paper, the Richardson constant is taken as the theoretical value. As a result, work function φ[eV] of a fluorescent lamp electrode halfway through its lamp life was measured and given as: φ=0.80+1.36×10-3T (computed from equation with T2) where T was the electrode temperature [K].Second, an attempt is made to measure a temperature-dependent work function taking a realistic value of the Richardson constant. The smaller the value of the Richardson constant, the smaller the estimated work function is. Finally, the paper examines how to take a realistic value of the Richardson constant.

Towards nanostructuring with femtosecond laser pulses

Detailed investigations of the possibilities for using femtosecond lasers for the nanostructuring of metal layers and transparent materials are reported. The aim is to develop a simple laser-based technology for fabricating two- and three-dimensional nanostructures with structure sizes on the order of several hundred nanometers. This is required for many applications in photonics, for the fabrication of photonic crystals and microoptical devices, for data storage, displays, etc. Measurements of thermionic electron emission from metal targets, which provide valuable information on the dynamics of femtosecond laser ablation, are discussed. Sub-wavelength microstructuring of metals is performed and the minimum structure size that can be fabricated in transparent materials is identified. Two-photon polymerization of hybrid polymers is demonstrated as a promising femtosecond laser-based nanofabrication technology.

One-dimensional heterostructures in semiconductor nanowhiskers

We report on the growth of designed heterostructures placed within semiconductor nanowhiskers, exemplified by the InAs/InP material system. Based on transmission electron microscopy, we deduce the interfaces between InAs and InP to be atomically sharp. Electrical measurements of thermionic emission across an 80-nm-wide InP heterobarrier, positioned inside InAs whiskers 40 nm in diameter, yield a barrier height of 0.6 eV. On the basis of these results, we propose new branches of physics phenomena as well as new families of device structures that will now be possible to realize and explore.

Anomalous behavior of the Fermi energy in heavily tin-doped InGaAs

We have measured the dependence of the Fermi energy on carrier concentration in Sn doped InGaAs at 4.2 K and 300 K. At 4.2 K the Fermi energy was measured by photoluminescence spectroscopy, and at 300 K it was deduced from transport measurements of thermionic emission. In both cases the dependence of the Fermi energy on the mobile electron concentration, measured by Hall effect, strongly deviates from standard theoretical predictions, and the deviation increases with concentration. The most striking observed anomaly is the near saturation of the Fermi level when the Hall concentration exceeds 1019 cm-3.

The suppression by pressure of negative differential resistance in GaAs/GaAlAs double barrier structures

High-pressure measurements of thermionic emission (TE) and of resonant tunneling in Ga1−xAlxAs/GaAs double barrier structures are reported, where x=1 or 0.33. For x=1, TE in a structure with a very narrow well yields a direct measurement of ∼150 meV for the Γ-X barrier height, and a shift of ∼−11 meV/kbar. For a structure with a well width of ∼70 Å and barriers of ∼40 Å, negative differential resistance (NDR) is observed, which is suppressed at a pressure of ∼8 kbar, when the height of the Γ-X barrier is approximately equal to the confinement energy of the state in the well. For x=0.33, and in samples with spacer layers, the same criterion for suppression of the NDR applies as for x=1. When spacer layers are absent, anomalies occur in the variation of the first NDR resonance with pressure, and for sufficiently large samples, the threshold for loss of NDR is much lower than expected. The anomalous behavior is related to the higher concentration of impurities in the barriers. The low-pressure threshold of the anomaly, and the dependence of the anomaly on sample size, suggest that impurity correlation may play a significant role in the suppression of NDR. At low pressures, or in the absence of anomalies, the pressure dependence of the peak and valley currents of all resonances which are always in the range −1% to −3%/kbar, indicates that the Γ profile controls the tunneling, through the pressure dependence of the effective mass.

Thermionic emission and photoluminescence studies of the energy states of GaAs/AlAs superlattices

We have performed thermionic emission measurements on doped GaAs/AlAs superlattices and photoluminescence measurements on a series of undoped GaAs/AlAs superlattices having different well widths. Using these techniques, we determined that the energy states in the GaAs wells are bounded by the Γ minimum of the AlAs barriers, and that the X minimum instead of the Γ minimum in the AlAs layer is the relevant barrier height for the current transport in our experiment. From the magnitude of the thermionic emission current, the coupling between the Γ and X states is shown to be approximately 100 times weaker than the Γ-Γ coupling, which is consistent with the theoretical tight-binding model.

Barrier tuning by means of a quantum, interface-induced dipole in a doping layer

We consider a sharply defined doping layer near the interface of a heterostructure on the narrow-gap side. It is shown that an interface-induced dipole moment results, whose magnitude depends on the quantum spread of the electronic charge. The result of thermionic emission measurement of the barrier height is compared with a self-consistent, nonparabolic subband calculation.

High-pressure studies of resonant tunnelling in a graded parameter superlattice and in double barrier structures of GaAs/AlAs

The authors report resonant tunnelling and thermionic emission measurements at pressures up to 9 kbar and temperatures down to 4.2 K, of p-i-n and n-i-n diodes containing a graded gap superlattice and double barrier structures, respectively, in which the wells are of GaAs and the barriers are of AlAs. The graded gap superlattice was grown by MBE and consists of five nominally 100 AA periods of GaAs/AlAs in which the 'mark-space' ratio of GaAs to AlAs is increased uniformly from 94 AA of GaAs, 6 AA of AlAs to 70 AA GaAs, 30 AA AlAs. The double barrier structures were grown by MOCVD with either approximately 70 AA wells and approximately 40 AA barriers or approximately 15 AA wells and approximately 50 AA barriers. The size of the NDR in the p-i-n structures increases with decreasing temperature as previously reported, the best samples exhibiting a ratio of maximum to minimum tunnelling current in excess of 2 at 4.2 K and 1 bar. On applying pressure, the size of the NDR at 4.2 K decreases slowly up to 5 kbar and more rapidly at higher pressures, almost vanishing by 8 kbar. A similar effect occurs in both directions of bias for the 43-72-40 AA double barrier structures, even though these have a ratio of maximum to minimum tunnelling current in excess of 10 at 4.2 K. In contrast, the 45-15-50 AA double barrier structures do not exhibit negative differential resistance, but thermionic emission measurements yield values for the barrier height, and its pressure dependence, of approximately 0.15 eV and -0.011 eV kbar-1 respectively. They take all these observations as evidence that X-like barrier states are strongly involved in the resonant tunnelling process, and that in general the pressure threshold for loss of NDR in samples grown in different ways, and with different ratios of peak to valley currents, is essentially a function only of well and barrier dimensions.

The thermionic emission and work function of U and UO2

Thermionic emission measurements have been used to determine the work function (φ) of pure and oxidized uranium samples between 1100 and 1300 K; Auger electron spectroscopy (AES) was used to verify the cleanliness and compositions of the samples. It was found that impurities present in ppm amounts in the bulk U segregated to the surface upon heating and had an appreciable effect on the zero-field emission currents as well as the slopes of the Schottky curves obtained at various temperatures. A combination of ion-sputtering and ultrahigh vacuum (UHV) annealing at high temperatures was successful in reducing the total impurity level on the hot surfaces to 5%. At this low concentration of impurities, well-behaved Richardson line plots were obtained with A=135 A cm−2 K−2 and φ=3.54 eV for pure U and A=128 A cm−2 K−2 and φ=3.19 eV for UO2. The Schottky coefficients for clean U approached their ideal values at fields &amp;gt;400 V/cm.

Measurement of work function in a cylindrical mirror electron spectrometer and application to characterization of thermionic electron emitters

Two techniques for the measurement of work function (WF) in a typical Auger electron spectrometer - the cylindrical mirror analyzer (CMA) - will be described and discussed. The first, utilizing shifts in the secondary electron emission threshold, has been more widely used by other workers. A means for computerization and discussion of the accuracy to be expected, as well as applications of this method to the adsorption of gases and metals on various substrates, will be given. Associated with this discussion will be observations on surface states and surface state spectroscopy. The second method for determining WF in a CMA is applicable in cases where a hot, electron emitting sample is to be studied; it does not appear to have been used in any previous studies. Results on WF variation with temperature from several dispenser cathodes will be used to illustrate this method which has the additional advantage over other thermionic emission measurements of not suffering from space charge limitations.