Ozone generator with a ceramic-based dielectric
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an ozone generator as described in Patent claim 1 (German Offenlegungsschrift No. 3,128,746). 
     2. Discussion of the Background 
     In the German Offenlegungsschrift No. 3,128,746 a dielectric which is applied in layer form to a tube-shaped or plate-shaped self-supporting metal body which forms one electrode is proposed for use in ozone generators. This dielectric is a few 100 μm thick and consists of a ceramic powder homogeneously distributed in a plastic material. Preferably, a phenylmethylpolysiloxane or a modified silicone is used as the plastic material and a ceramic having a linear temperature dependence and a relatively high dielectric constant such as is used inter alia for the manufacture of ceramic capacitors is used for the ceramic powder. 
     SUMMARY OF THE INVENTION 
     This invention relates to the facts that with an increase in the proportion by weight of ceramic powder, the dielectric constant of the dielectric layer also increases, but if more than 50% by weight of ceramic powder is used, the dielectric strength of the dielectric decreases considerably. 
     Consequently limits are obviously imposed on a per se desirable increase in the dielectric constant to raise the power density of the ozone generator. In addition to this, the dielectric strength values achievable are related to the dielectric strength of the dielectric itself. Extensive investigations by the Applicant have shown that this &#34;short-term dielectric strength&#34; is of lesser importance in the operation of an ozone generator. Of greater importance is the behaviour of the dielectric layer when permanently loaded by the discharge attack in the discharge gap of the ozone generator. This characteristic of the dielectric layer, described here as &#34;long-term dielectric strength&#34;, is orders of magnitude lower than the &#34;short-term dielectric strength&#34;. As a result of the discharge attack, small and very small components are constantly dislodged from the surface of the dielectric. At these defect positions the dielectric is then locally subjected to still greater stress, which results in the further dislodging of particles and ultimately leads to breakdown even at low field strengths. 
     Our invention, as described in the patent claims, is based on the objective of providing an ozone generator whose dielectric exhibits a substantially higher long-term dielectric strength and makes possible proportions by weight of the dielectric filling material of over 50% by weight. 
     In this connection the invention is based on the following new knowledge. 
     If larger grains of a corona-resistant material are included in the dielectric layer in addition to the binding agent and the fine-grained ceramic powder, then fine-grain powder particles and binding agent are indeed dislodged in the surface region of the layer by the discharge attack. The electric field cannot, however, randomly penetrate the holes so formed between the &#34;coarse&#34; grains because an electric discharge is no longer taking place there. A mechanical analogy to this would be the &#34;paving stone effect&#34;. In the case of paved roads and paths, limits are imposed on the erosion on the filling material between the paving stones by the paving stones themselves. 
     Barium titanate is preferably used as the ceramic powder. In addition to a mixture of glass pellets and fused curundum or fused corundum alone, in particular TiO 2  grains of appropriate diameter are also suitable as coarse-grain dielectric components of the dielectric layer. Since TiO 2  is only available commercially as a fine-grain powder, the coarse grains are obtained by crushing sintered TiO 2 . 
     In particular anhydride-hardened epoxy resin is suitable as a hardenable plastic material. 
     The dielectric is applied, for example, by vacuum casting, by the pressurised gelling process or by the displacement method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below in more detail by reference to the drawing. 
     In the drawing 
     FIG. 1 shows a section through an ozone generator on a highly magnified scale after a prolonged period of use, 
     FIG. 2 shows a diagrammatic representation to illustrate the mode of operation of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1 a first metallic electrode is denoted by 1, and a second metallic electrode by 2. The second electrode 2 exhibits a dielectric layer 3 on the surface facing the first electrode. Between the dielectric layer 3 and the first electrode 1 there extends the discharge gap 4, typically 0.6-2 mm wide, of the ozone generator. 
     The dielectric layer 3 contains a multiplicity of TiO 2  grains 5 more or less uniformly distributed and barium titanate grains 6 in between. Both types of grain are included in a thermosetting, preferably anhydride-hardened, epoxy resin with a relative dielectric constant of ε r  =3.5. The thickness of the dielectric layer 3 is between 1 and 5 mm, preferably 2.5 to 3 mm. The barium titanate powder exhibits a grain size of less than 1 μm and has a relative dielectric constant of ε r  =2,000. 
     The coarse grains consist of TiO 2  with grain sizes between 5 and 100 μm with a relative dielectric constant of ε r  =100. These are obtained by sintering fine-grain TiO 2  followed by crushing. 
     With a layer constructed in this manner, which exhibits approximately 42% by volume of artificial resin, approximately 20% by volume of barium titanate powder and approximately 38% by volume of coarse-grain TiO 2 , a relative dielectric constant for the entire layer of approximately ε r  =30 was obtained. The long-term dielectric strength was over 2,000 V/mm. 
     Comparable values are provided by a dielectric layer 3 containing a mixture of glass pellets and fused corundum as the coarse-grain component of the dielectric layer. 
     FIG. 2 illustrates how the higher long-term dielectric strength compared with that known can be explained: 
     With increasing operating time the fine-grain components are dislodged from the original dielectric surface 7 by the discharge attack (corona filaments 8). This process also takes place in the gap between two neighbouring TiO 2  grains 5. This material cannot, however, be dislodged at an arbitrary depth, since an electric field (symbolised by arrow 9) cannot penetrate a hole to an arbitrary depth and consequently an electric discharge can no longer take place there. The projecting coarse TiO 2  grains 5 are not attacked by the discharge. 
     An ozone generator according to the invention exhibits the following advantages: 
     Without additional circuit-engineering costs on the supply side, power densities of over 20 kw/m 2  can be achieved, given a small gap width and double cooling of both electrodes; 
     Very small (mechanical) tolerances can be maintained which are virtually independent of the technique of application of the dielectric layer, e.g. cast-coating; 
     Optimum heat transfer between dielectric and heat sink; in contrast to the classical glass tube with an inner electrode, the liquid coolant can be brought directly into contact with the electrode supporting the dielectric; 
     A special internal contact is unnecessary; 
     The power factor (cos φ) is better; 
     The invention can be used both in tube and in plate ozone generators.