Eur. Phys. J. Appl. Phys. (2015) 69: 30801 DOI: 10.1051/epjap/2015140468 THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS Regular Article Preheated ignition and work function studies on alkaline earth metal oxides coated tungsten electrodes of fluorescent lamps Reinhard Langer 1,2, a , Farrukh Dar 1,2 , Achim Hilscher 2 , Siegfried Horn 1 , and Reinhard Tidecks 1 1 Institut f¨ ur Physik, Lehrstuhl f¨ ur Experimentalphysik II, Universit¨at Augsburg, Universit¨atsstr. 1, 86159 Augsburg, Germany 2 OSRAM AG, Berliner Allee 65, 86136 Augsburg, Germany Received: 27 November 2014 / Received in final form: 15 January 2015 / Accepted: 22 January 2015 Published online: 12 March 2015 – c  EDP Sciences 2015 Abstract. This paper describes measurements on the alkaline earth (Ba, Sr, Ca) oxide layers utilized as emitting material in fluorescent lamp electrodes. In a first approach we compared the emission capabilities of the different materials (BaO, SrO and CaO coatings on tungsten filaments) by their ability to facilitate the start of a lamp. In a second investigation the work function was measured quantitatively by means of a Kelvin probe. Basically, the results of both measurements coincide. The impact of sample geometry on measurement results of a Kelvin probe applied for work function measurements of lamp electrodes are discussed in detail. 1 Introduction The work function of the electrodes in a fluorescent lamp is the key factor for its lifetime. When the work function is low, the emitting material is lost predominately by evapo- ration, sputtering is very low, if there is any [1]. Therefore, life time is very long. The work function of the polycrys- talline tungsten filament (4.55 eV, Ref. [2]) of a typical lamp electrode is lowered to less than 2 eV by a coating of (Ba, Ca, Sr)-oxides [3], referred to as “emitter”. Though Wehnelt [4] found in 1904, that these oxides reduce the work function, there is still no coherent description, how the emission works in detail [5]. Utilizing a semiconductor model basic properties of the work func- tion lowering mechanism could be explained quite suc- cessful. Nevertheless, the adsorption of atoms might be even more important (cp. Sect. 5), as the work function is specially sensitive to variations in the dipole layer on the surface [6]. We used two methods to investigate these emitters. The first method, referenced to as “preheated ignition study”, allows only a qualitative comparison between their emission capabilities, but has the advantage that usual commercial lamps with standard electrode design can be used. The second method, applying a Kelvin probe, allows the determination of absolute work function values, but specially prepared samples are necessary (cp. Sect. 2). a e-mail: reinhard.langer@physik.uni-augsburg.de 2 Sample preparation In Table 1 the emitters under investigation are introduced. Emitter C and D differ mainly in the CaO content and contained approximately 5% ZrO 2 , what is common in lamp production [7]. For the preheated ignition study the samples were fabricated in the same way as in the usual lamp production, i.e., the corresponding carbonates were brought onto the tungsten coil and the breakdown into the oxides was performed by electrical current in the lamp [7]. As with the Kelvin probe electrodes prepared in this way could not be investigated (cp. Sect. 4.2), the way of sample preparation was changed. A bunch of tungsten coils was pressed together to form a mesh. The carbonate suspension was brought onto this mesh and the latter was sealed into a glass tube filled with inert gas. The heat- ing up and the breakdown was performed by bringing the sealed tube into a radio frequency field. 3 Preheated ignition study 3.1 Method In steady state a fluorescent lamp is operated in an arc discharge, i.e., the temperature in the attachment point of the arc on the electrode is so high, that thermionic emission of electrons dominates. It is well known, that the thermionic emission current density j e depends on 30801-p1