Fluorescent lamp cathodes: Voltage-controlled preheating characteristics

Summary form only given. We have developed a new type of field enhanced thermionic cathode. This cathode is capable of emitting much higher electron current, because of the strong field enhancement factor induced by incorporating needle like geometrical feature into the cathode structure. Field enhancement effect is usually negligible for conventional thermionic cathodes. However, in this case significant field enhancement thermionic emission is induced by introducing sharp carbon nanotubes tips. The cathode consists of a metal coil/substrate with carbon nanotubes grown on top of its surface by plasma enhanced CVD technique. The carbon nanotubes are further coated with thermionic emission materials (BaO/SrO/CaO) - a typical thermionic emission material system used in fluorescent lighting and many other applications. The oxides are coated on carbon nanotubes by magnetron sputter deposition and spin coating techniques. During operation, the cathode is heated up to a thermionic emission temperature by flowing an electric current through the metal coil. The emission current and current density from this cathode are found at least an order of magnitude higher than conventional thermionic cathode coated with same emission materials and operated at same temperature. We will present comparison results on electron emission for three different cathodes: 1) oxide coated carbon nanotube cathode, 2) conventional oxide cathode, and 3) carbon nanotube cathode

As one of the alternatives Th-W materials, La-W thermionic emission materials appeared in recent years. Although this emission material has good emissivity, it has not been used commercially because of its poor emission stability. Properties of a material depend on composition and microstructure. In this paper, a fine grain tungsten emission material, activated with nanoscale oxide, was fabricated by freeze-drying and spark plasma sintering, which differs completely compared to the conventional process. Emphasis has been concentrated on the relationship between surface composition and electron emission performance.

The history of electron emission is reviewed from a standpoint of the work function that determines the electron emission capability and of applications in the fields of scientific instruments and displays. For years, in thermionic emission, a great deal of effort has been devoted to the search for low work function materials with high melting temperature, while reduction of the local change in time of the work function rather than the work function itself has been the main issue of field emission investigations. High brightness and long life are the central targets of emission material investigations for scientific instrument applications, while high current density and low power consumption are the guiding principles for display applications.In most of the present day industries, thermionic emission materials are exclusively used in such fields requiring high current and high reliability as cathode ray tubes, transmission and receiving tubes, x-ray sources and various electron beam machines. Field electron emission sources, however, since applied to high resolution electron microscopes in the 1970s have recently become dominant in research and development in the fields of scientific instruments as well as in the fields of various electron tubes and beam machines.The main issue in this report is to analyse the work function on the atomic scale and thereby to understand the fundamental physics behind the work function, the change in time of the local work function leading to field emission current fluctuation and the relationship between microscopic (on atomic scale) and macroscopic work functions.Our attempt is presented here, where the work function on the atomic scale is measured by utilizing a scanning tunnelling microscopy technique, and it is made clear how far the local work function extends its influence over neighbouring sites. As a result, a simple relationship is established between microscopic and macroscopic work functions.

High‐temperature thermionic electron emission from different types of individual suspended carbon nanotubes (CNTs) in a homemade chip is studied. When collecting emission electrons with an external electrode, it is found that the collecting electric field influence the transport current in the CNTs, and the thermionic electron emission deviated from the traditional thermionic electron emission model and cannot reach “saturation”. This effect is analyzed for single‐walled, double‐walled, multiwalled‐CNT and CNT bundles. It is found that this effect show correlation with the conductivity and diameter of the CNT. With the increase in metallicity, the influence of collecting electrode is gradually weakened. Also, the effect shows a negative correlation with the diameter of CNTs. An equation is proposed to describe the measured thermionic emission curve and determine the “effective” work function of the CNT.

Poly(1′,4′-phenylene-1",4"-(2"-(2""-ethyl-hexyloxy)) phenylene-1",4"-phenylene-2,5-oxadiazolyl) (PPEPPO) was synthesized and its electrochemical characteristics was investigated as electrode material in redox supercapacitor. The cyclic voltammetry (CV) shows there was scarcely a redox reaction and further suggests n-doping is difficult to occur in this system. However, the discharge curve between 3.0 to 0.01 V is continuously decreased like a straight line, similar to the discharge pattern of EDLC. The initial specific discharge capacitance is ∼6.4 F/g, while the specific capacitance of 1000th cycle is ∼ 0.1 F/g. The PPEPPO can be used as the electrode of supercapacitor, emissive material, as well as charge-transporting material in polymer LED.

DC lamp voltage monitoring by microcontroller ATmega 328P through wireless zigbee is a monitoring tool that uses a DC lamp AVR microcontroller as a data processor , voltage and current sensors on the lights to detect the lamp current and voltage power supply , monitoring DC lights will transmit data to the monitoring in the form of a PC (Personal Computer) with the help of Visual Basic software, data transmission through wireless zigbee. Monitoring lamps DC serves to detect damage to the lamp and power supply, monitoring can be used to monitor street lighting that uses a DC lamp. Monitoring can monitor the DC lamp lights up to 50W and the power supply voltage is 12 VDC and can be informed remotely by using zigbee as a communication medium. Keywords: zigbee, lights, monitoring, ATmega

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.

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

In order to enhance output power and prolong lifetime of the high-power magnetron tubes, a novel Y 2 Hf 2 O 7 Ceramic cathode had been developed. The thermionic emission and lifetime characteristics of the Y 2 Hf 2 O 7 cathode had been measured. The results show that the cathode can provide 0.15A/cm2, 3.5A/cm2 current density for the space charge limitation at 1300 Cbr, 1600 °C br respectively under 300V anode voltage. The lifetime of the cathode is more than 4100 h with an initial load of 0.5A/cm2 at 1400Cbr.

Scandate cathode as a thermionic electron emission material has attracted broad interest owing to its high electron emission capability. Despite the large number of studies on the electron emission of the scandate tungsten cathode, there is a lack of understanding of the performance of scandium oxide incorporated in other refractory metal matrix. In this work, by applying first-principles calculations, we study the role that Sc2O3 nanocluster plays in enhancing the electron emission of the Hf cathode (labeled as Sc2O3@Hf). We find that Sc2O3@Hf is both thermally and mechanically stable, exhibits complex surface structures, and possesses inverse relationship between surface energy and work function. Moreover, the work function of the Hf (0001) surface can be reduced to 3.50 eV due to the change in the vacuum level when Sc2O3 nanocluster is doped, while the work function of the Hf (101¯2) surface can be reduced to 3.39, 3.39, and 3.03 eV when doped with Sc2O3, Y2O3, and CeO2 clusters, respectively.

Along with the development of cold field emissi on technology, more and more microscopes equipped with cold field emission gun (C-FEG ) are applied in scien tific research. Thanks to the lower working temperature, the service time of emitter has been ex tended. In the meantime, this technology allows to emit electrons from smaller emission radius compared to thermionic emission and Schottky emission [1]. Based on this property, higher curre nt density, higher brightness, be tter special coherence and lower energy distribution of the emission can be expected, wh ich also means that a better electron beam can be obtained [2]. For the further improvement of cold field emission, new emitter materials have been explored. As a rolled-up graphe ne sheet, carbon nanotubes (CNTs) have the similar property, for instance, low work function, high mechanic performan ce, high melting point, high carrier mobility [3-4]. In addition, CNTs have small apex radius curvature for its quasi-one -dimensional character at nanoscale. Therefore, CNTs are capable of working under the operating condition of cold field emission [5]. Furthermore, carbon nanocones are considered as candi date as well [6]. Significant improvements of such kind of carbon nanocones are demonstrated by repl acing the regular tungste n tip in transmission electron microscope [7]. For a higher performance of the cold field emission application, the electronic band structure of carbon nano-object can be modulat ed through the introduction of heteroatoms and a lower work function can be expected [8,9]. Our work is devoted to th e doping of these carbon nanostructures by nitrogen and/or boron, and to the evaluation of these doped carbon nano-objects by a characterization for the cold field emission application.

In this work the absolute intensity of barium lines during dimming operation in the vicinity of fluorescent lamp electrodes is investigated using the method of atomic emission spectroscopy. Investigations of fluorescent lamps (FL) are often focused on the electrodes, since the lifetime of the lamps is typically limited by the electrode lifetime and durability. In general, a commercial electrode system consists of a tungsten coil coated with a work function reducing emitter mix of alkali oxides, such as BaO, SrO and CaO. One of the main issues with dimming is a reduction in lamp life when the additional heating of the electrodes is not optimal, causing not-optimal electrode temperatures. There is a need for design rules for dimming to reach good lamp life, based on good additional heating. Such dimming design rules can be derived from measurements of the absolute intensities of both neutral (Ba I) and ionised (Ba II) barium lines, as these are key parameters to describe the evaporation and the sputtering of the emitter material. During steady state operation free barium is produced, which transports to the surface by diffusion through the coating mass. Barium escapes from the emitter during the course of lamp operation due to sputtering (primarily during starting and dimming), where the discharge is sustained by secondary electron emission from the (cold) electrode, and due to evaporation (primarily during steady-state), where the discharge is sustained by thermionic emission from the (hot) electrode. In the first type of experiment, the atomic emission diagnostic is used for the detection and measurement the intensities of the neutral (Bal-553.5 nm) and ionised (Ball-455.4n m) barium. The FL is dimmed for a range of discharge currents and auxiliary coil heating currents. It is seen that there is a threshold discharge current in which the behaviour of barium intensity with respect to current is markedly different. In a second type of experiment, we can estimate the relative intensities for Ba I and Ba II from the relative area under their line of absolute intensities. From these two measurements we show that the Ba loss can very easily be reduced by appropriate auxiliary coil heating.

The purpose of the work performed was to demonstrate the feasibility of a novel bi-layer selective emitter. Selective emitters are incandescent radiant bodies with emissivities that are substantially larger in a selected part of the radiation spectrum, thereby significantly shifting their radiated spectral distribution from that of a blackbody radiating at the same temperature. The major research objectives involved answering the following questions: (1) What maximum VIS/NIR radiant power and emissivity ratios can be attained at 2650 K? (2) What is the observed emitter body life and how does its performance vary with time? (3) What are the design tradeoffs for a dual heating approach in which both an internally mounted heating coil and electrical resistance self-heating are used? (4) What are the quantitative improvements to be had from utilizing a bi-layer emitter body with a low emissivity inner layer and a partially transmissive outer layer? Two approaches to obtaining selective emissivity were investigated. The first was to utilize large optical scattering within an emitter material with a spectral optical absorption that is much greater within the visible spectrum than that within the NIR. With this approach, an optically thick emitter can radiate almost as if optically thin because essentially, scattering limits the distance below the surface from which significant amounts of internally generated radiation can emerge. The performance of thin emitters was also investigated (for optically thin emitters, spectral emissivity is proportional to spectral absorptivity). These emitters were fabricated from thin mono-layer emitter rods as well as from bi-layer rods with a thin emitter layer mounted on a substrate core. With an initially estimated energy efficiency of almost three times that of standard incandescent bulbs, a number of energy, economic and environmental benefits such as less energy use and cost, reduced CO{sub 2} emissions, and no mercury contamination was initially projected. The work performed provided answers to a number of important questions. The first is that, with the investigated approaches, the maximum sustained emitter efficiencies are about 1.5 times that of a standard incandescent bulb. This was seen to be the case for both thick and thin emitters, and for both mono-layer and bi-layer designs. While observed VIS/NIR ratios represent improvements over standard incandescent bulbs, it does not appear sufficient to overcome higher cost (i.e. up to five times that of the standard bulb) and ensure commercial success. Another result is that high temperatures (i.e. 2650 K) are routinely attainable without platinum electrodes. This is significant for reducing material costs. A novel dual heating arrangement and insulated electrodes were used to attain these temperatures. Another observed characteristic of the emitter was significant grain growth soon after attaining operating temperatures. This is an undesirable characteristic that results in substantially less optical scattering and spectral selectivity, and which significantly limits emitter efficiencies to the values reported. Further work is required to address this problem.

Electron emission has been observed when tungsten and molybdenum targets are subjected to focussed CO2 laser radiation. The characteristics of thermionic emission for a variety of excitation conditions are reported. The electron temperature is found to be for an incident power of 40 W cw. Direct energy conversion processes are discussed. Short circuit currents of 2–7 μA and open circuit voltages of 0.1–0.4 V have been observed.

[Ca 24 Al 28 O 64 ] 4+ (4e − ) (C12A7:e − ) has excellent potential in electric propulsion owing to various advantages, including low work function, low working temperature, and stable chemical properties. However, increasing electron concentration enhances the thermionic emission property of C12A7:e − . This study used a chemical method—spark plasma sintering to synthesize Zr‐doped C12A7:e − (C12(A 1 − x Z x )7:e − , where x = 0 to 0.14). The study results show that C12(Al 0.86 Zr 0.14 )7:e − maximum injected electron concentration ( n ), conductivity ( σ ), electron mobility ( μ n ), and zero‐field emission current density ( J o ) improved to 2.4 × 10 21 /cm 3 , 1507 S/cm, 5.2 cm 2 /V·s, and 2.4 A/cm 2 , respectively. The emission current density of the C12(Al 0.86 Zr 0.14 )7:e − could be maintained at about 2.9 A/cm 2 under 1100°C, 4000 V, and 600 min. Theoretical calculation results prove that Zr doping broadens the cage conduction band (CCB) of C12A7:e − and reduces the bandgap between the CCB and the frame conduction band. In addition, Zr doping increases the electronic density of states near the Fermi level, and the density of states presents a continuous state. This helps to increase the injected electron concentration and enhance the electrical transport and thermionic emission characteristics of the sample. The calculation results provide theoretical support for the experiment.