A fully sealed luminescent tube based on carbon nanotube field emission

Abstract

A fully sealed luminescent tube of 40 cm length and 4 cm diameter based on carbon nanotube field emission is demonstrated. The device shows a homogeneous illumination over the whole length and circumference of the tube and reaches the luminance of conventional fluorescent tubes while being mercury-free, continuously dimmable and with a high illuminance capability. The realization has been made possible with the development of a chemical vapor deposition method to grow nanotubes homogeneously on long metallic wires, which provides an additional possibility to control the mean length and density of the emitters. This control has proven to be of utmost importance as it makes possible to adjust the emission voltage and emission site density needed to reach the target intensity and specifications of the device.

Introduction

Carbon-based materials, and especially carbon nanotubes [1], have been used recently as cold electron emitters [2] in a variety of devices aiming at lighting applications [3], [4], [5], [6]. In such applications, the electrons are accelerated towards a layer of phosphorescent material, which converts the kinetic energy of the electrons into visible light. Most of these devices, which are similar in shape to the traditional vacuum tubes, consist in a field emission diode or triode where a film emitter illuminates a flat phosphor layer of ∼20 mm diameter. Such elements are usually designed to be used as pixel elements in giant (outdoor) screen displays, instead of existing solutions based on thermoelectronic emission or on semi-conductor diodes. Several industrial companies have demonstrated that such elements can be realized in the three basic colors with carbon-based materials (e.g. FEPET, a subsidiary of SI Diamond Technologies [3]) or carbon nanotubes (e.g. Ise Electronics Corp. of Mie, Japan). In particular, the high brightness luminescent elements of Ise Electronics Corp. [7] have received the silver medal of the ‘Displays of the Year Awards 2000’ of the Society for Information Display, for ‘the first commercial product that uses field emission from carbon nanotubes’. In these elements, the phosphor-coated glass anode is flat and shows a surface of a few cm², which means that a large number of elements is needed for a practical application. While this constraint is usual in giant displays, it poses a severe drawback for general lighting devices.

Our approach is different and aims specifically at providing an alternative to the usual fluorescent tubes. Fluorescent tubes are based on an electric discharge in a rare gas with the addition of a small amount of mercury. Excitation of the mercury vapor produces UV photons that are converted to visible light by a phosphor layer deposited on the inner surface of the glass tube [8]. The main drawbacks of such tubes are that the light intensity is limited to ∼10,000 cd/m², that their dimming capability is poor, and that they contain mercury, a toxic substance which will be forbidden in some electronic devices in the European Union from 2008 on. An alternative solution to usual fluorescent tubes would therefore have a high market potential, provided that it corrects the drawbacks of the usual tubes while remaining cheap, of simple technology and energetically efficient. We demonstrate here a luminescent tube of 40 cm length based on carbon nanotube field emission that is mercury-free, continuously dimmable and has a high illuminance capability.

To realize a luminescent tube based on electron emission, the phosphor-coated inner surface of a glass tube has to be bombarded homogeneously with electrons, as schematized in Fig. 1(a). This implies that a conductive layer has to be deposited below or on the phosphor to efficiently remove the electrons, and that the cathode has to be cylindrical, as it is not possible to provide a current density that is constant over the whole circumference of a cylindrical anode with emitters deposited on a flat surface. One solution would be to use a thin wire, where the electrons are extracted either by heating or by field emission [9], but this leads to extremely poor homogeneity. Furthermore, field emitters like Spindt tips and carbon nanotubes are traditionally deposited on flat Si substrates which has prevented up-to-now the realization of such tubular elements. The only possibility is therefore to deposit field emitters on a non-planar substrate.

We have shown recently that carbon nanotubes can be deposited on wires and rods by thermal chemical vapor deposition (CVD) of C₂H₂ at 720 °C on a variety of materials [10], [11]. In thermal CVD, a hydrocarbon gas is injected in a flow tube that crosses the hot zone of a tubular oven. The nanotube growth is activated by a transition metal catalyst which decomposes the C₂H₂ and promotes the growth of well-graphitized tubular nanostructures, and Fig. 1(b) shows carbon nanotubes grown by this technique on a thin wire. We use Kanthal (a Fe–Al–Cr alloy) as support material, as it allows (after annealing in air at 1000 °C over 12 h) a selective control of the growth by the deposition of a Fe-based catalyst (20 mM of Fe(NO₃)₃·9H₂O in ethanol) that is delivered onto the metallic wire by dipping [11]. This control permits to tune the nanotube density [12] and to grow nanotubes only on precise locations on the cathode. We could also demonstrate with such structures that field emission in the cylindrical geometry of Fig. 1(a) is not only possible, but very efficient [10] (to our knowledge, this was the second demonstration after the one by Millikan and Eyring in 1926 [9]), which makes possible to realize luminescent tubes. Fig. 1(c) shows an element of 10 cm length yielding 10,000 cd/m² at 7.5 kV. The next step after this achievement was of course to realize a fully sealed element with the specifications summarized in Table 1: in particular, the length had to be increased to at least 40 cm.