Abstract:
A resistor element with a ceramic body that has PTC properties is specified. At least one main surface of the ceramic body has an arrangement of depressions.

Description:
This application is a continuation of co-pending International Application No. PCT/DE2007/001293, filed Jul. 19, 2007, which designated the United States and was not published in English, and which claims priority to German Application No. 10 2006 033 691.7 filed Jul. 20, 2006, both of which applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     An arrangement with particles of PTC material that are distributed in a binder is known from German patent publication DE 3107290 A1. A flexible element in ribbon form is known from German patent publication DE 8309023 U1. 
     SUMMARY 
     In one aspect, the present invention specifies a resistor element that is characterized by high electrical and thermal conductivity. 
     For example, a resistor element with a ceramic body of ceramic that has PTC properties is specified. The abbreviation PTC stands for “positive temperature coefficient.” At least one main surface of the ceramic body has an arrangement of depressions. 
     Preferably, the first main surface of the ceramic body has an arrangement of first depressions and the second main surface of the ceramic body has an arrangement of second depressions. 
     The main surfaces of the ceramic body, including the surface of the depressions, are preferably coated with an electrode layer. Each electrode layer forms an electrode surface. The resistance of the resistor element will be lower, the greater the electrode surface and the smaller the distance between the electrode layers. These parameters are directly dependent on geometric parameters such as the depth and width of the depressions and the distance between the depressions. By adjusting the electrode area and the spacing between electrode layers as illustrated below, it is possible to achieve a specified resistance value for the specified size of the resistor element. 
     Through the depressions it is possible, in particular, to enlarge the effective electrode surface of the ceramic body and thus to lower the resistance value of the resistor element compared to a design without depressions. Through the depressions it is additionally possible to reduce the distance between two oppositely lying electrode surfaces of the resistor element. Through the increase of the electrode surface it is also possible to achieve an especially small resistor element with high heat dissipation. Low resistances and high heat dissipation are also achieved by small spacings of the depressions. 
     The first (and second) depressions preferably have the shape of slots or grooves that run parallel to each other. However, the depressions can also be designed as blind holes. A regular arrangement of uniformly designed depressions is preferred. 
     The second depressions can run parallel to the first depressions. However, the second depressions can also run across, in particular, perpendicularly or obliquely, to the first depressions. 
     The depressions can have any cross section. In particular, the side walls of the depressions can run perpendicularly or obliquely to the main surfaces of the resistor element or can be curved. The depressions can also have steps. 
     The depth of the depressions preferably is greater than their width. The depth of the depressions can, for example, be at least twice the width. The depth of the depressions is preferably at least 20% of the thickness of the ceramic body. The depth of the depressions can even exceed 50% of the thickness of the ceramic body. The first and second depressions can have the same depth. However, in principle, they can also have depths that differ from each other. 
     In an advantageous variation, the second depressions are staggered with respect to the first depressions (in a top view). In this case the ceramic body has a serpentine cross section. In this variation it is possible to form particularly deep depressions, the depth of which can exceed half the thickness of the ceramic body. 
     The staggered first and second depressions can overlap with respect to the direction of the thickness of the ceramic body (in a side view) so that they intermesh in a central region of the ceramic body. In this case, the first and second depressions are alternatingly arranged in the central region of the ceramic body. The depth of the depressions in this case exceeds half the thickness of the ceramic body. 
     In another variation, the second depressions can (in a top view) lie opposite the first depressions. In this case, the depth of the first and second depressions will be smaller than half the thickness of the ceramic body. 
     The depressions can at least partially be filled with a filler material, whose thermal conductivity exceeds that of the material of the ceramic body. In this way it is possible to create heat sinks in the ceramic body which improve the dissipation of heat of the resistor element to the environment, i.e., to an object. 
     The filler material can be an electrically insulating material. However, the filler material can also be electrically conductive. 
     The ceramic body is preferably a solid, rigid, sintered body. BaTiO 3  is suitable as the base material for the ceramic body. The ceramic body is preferably made as a plate. The depressions can be produced in a sintered ceramic body as indentations. After the formation of the depressions, the main surfaces of the ceramic body are metalized to form the electrode layers. However, there is also the possibility of making the depressions in a ceramic body that has not yet been sintered and to subject the ceramic body to sintering with the depressions already formed. 
     The electrode layers can in each case be deposited, for example, in an electrolytic process. However, they can also be applied by sputtering, evaporation or as a metal paste and fired onto the ceramic body. Combinations of these electrode technologies are also possible to produce particular sequences of layers. 
     Resistor elements put together in this way are preferably provided with electrical terminals for supply of current, where the mechanical design can correspond to any radially contacted or SMD-capable element. The formation of these elements can also involve coating with insulating materials or encapsulation in plastics. A number of resistor elements can be encapsulated together. These resistor elements can also be combined with at least one cover layer that lies flush, the thermal conductivity of which preferably exceeds that of the material of the ceramic body. This cover layer can be electrically conductive and can be suitable as a contact for the supply of current. The cover layer can also be designed as a composite that includes an electrically conductive partial layer and an electrically insulating partial layer. 
     The resistor elements can also be arranged without a premade connection to the cover layers so that the electrical and thermal contact to these layers can also take place later. A number of resistor elements mechanically connected to each other can be used together in one arrangement. These resistor elements are preferably electrically connected to each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The resistor element will now be explained by means of drawings, which are schematic and not to scale. Here: 
         FIG. 1  shows a resistor element with an arrangement of depressions on the two main surfaces of the ceramic body; 
         FIG. 2  shows the resistor element as in  FIG. 1  with the depressions filled with a filler; 
         FIG. 3  shows the resistor element as in  FIG. 2 , which is arranged between two cover layers; 
         FIG. 4  shows the resistor element as in  FIG. 2  in an SMD embodiment; and 
         FIGS. 5A-5F , collectively  FIG. 5 , shows various examples of the arrangement of the depressions. 
     
    
    
     The following list of reference symbols can be used in conjunction with the drawings:
           1 ,  1   a ,  1   b  Ceramic body     10  Central region of ceramic body     21  First depressions     22  Second depressions     3  Filler material     41  First cover layer     42  Second cover layer     51 ,  52  Electrical terminal     61  First electrode layer     62  Second electrode layer       

     DETAILED DESCRIPTION 
       FIG. 1  shows a resistor element with a ceramic body  1 . The ceramic body  1  has first depressions  21 , which are arranged on the first main surface (top), and second depressions  22 , which are arranged on the second main surface (bottom). As in the variation in  FIG. 2 , these depressions are preferably filled with a filler material  3 , which has better thermal conductivity than ceramic body  1 . 
     A first electrode layer  61  is arranged on the top of the ceramic body and a second electrode layer  62  is arranged on the bottom. The electrode layers  61  and  62  also coat the surface of the depressions  21  and  22 . 
     The second depressions  22  are laterally offset, or staggered, with respect to the first depressions  21 . The first and second depressions  21  and  22  are not connected to each other. The depth of the depressions  21  and  22  shown in  FIGS. 1 to 3  is preferably roughly half the thickness of the ceramic body  1 . A design of the depressions  21  and  22  with this sort of depth is particularly possible when: 
     a) the distance between two successive first depressions is greater than the width of the second depressions; and 
     b) the distance between two successive second depressions is greater than the width of the first depressions. 
     Other variations of depressions  21  and  22  with respect to depth and shape are illustrated in  FIGS. 5A through 5F . 
     In the variation in  FIG. 3  the ceramic body  1  is arranged between two cover layers  41  and  42 . The ceramic body  1  is preferably firmly bonded to the cover layers  41  and  42 , for example, glued. 
     The resistor element shown in  FIGS. 1 to 3  is suitable for use, for example, as a heating element. 
       FIG. 4  shows the resistor element in accordance with  FIG. 2 , having electrical terminals  51  and  52  extended to the bottom of the resistor element. Such a resistor element is a surface-mountable element or SMD element. The abbreviation SMD stands for “surface mounted device.” The resistor element shown in  FIG. 4  can be mounted on a circuit board and is a possibility, in particular, for current protection applications. 
     The resistor element can alternatively be designed as a wired element, i.e., with wire terminals. 
     The depth of the depressions  21  and  22  shown in  FIG. 5A  is greater than half the thickness of the ceramic body  1 , so that the first depressions partially intermesh and overlap in a central region  10  of the ceramic body. As in the variation in accordance with  FIG. 1  the ceramic body has a serpentine cross section. 
     Depressions  21  and  22  that are especially deep have the advantage that this results in an especially small distance between the electrode layers  61  and  62  and thus the resistance of the resistor element can be reduced. 
     The depth of the depressions  21  and  22  shown in  FIGS. 5B and 5C  is set to be smaller than half the thickness of the ceramic body  1 . In  5 C the second depressions  22  lie directly opposite the first depressions  21 . The remaining thickness of the ceramic body between depressions  21  and  22  is selected so that it is sufficient for stability of the resistor element. 
       FIG. 5D  shows a resistor element that has an arrangement of depressions  21  only on one side. 
     The depressions  21  and  22  of the resistor elements shown in  FIGS. 1 through 5C  have a rectangular cross section. The cross section of the depressions  21  and  22  can, alternatively, be rounded as in  FIG. 5D , have obliquely running side walls as in  FIG. 5E , or be V-shaped as in  FIG. 5F .