PTC-RESISTOR

The present invention relates to PTC-resistors having a base body comprising a ceramic material with a positive temperature coefficient of the ohmic resistance, at least in a certain range of temperature.

BACKGROUND OF THE INVENTION

The rise of the electric resistivity p as a function of the temperature follows a logarithmic curve in a certain temperature range. PTC-resistors may be produced in the form of disks with a circular, quadratic or rectangular shape. Such PTC-resistors are suitable for a wide range of applications, in particular including overcurrent protection devices, switches and additionally as heaters.

PTC-resistors can be fabricated by dry pressing of a granulate. The variety of possible shapes of such PTC- resistors with a base body being manufactured by dry pressing is strongly restricted to very simple geometric structures such as disks like those mentioned above.

SUMMARY OF THE INVENTION

A PTC-resistor is described having a base body comprising a ceramic material with a positive temperature coefficient of the ohmic resistance at least in a certain temperature range.

The base body mainly extends along a median layer. In addition, the base body may also have an extension perpendicular to the median layer.

The base body is confined by different surfaces whereby at least one of the surfaces is configured to electrically contact the base body.

The area of the at least one surface is larger than the area of the parallel projection of the base body in a direction perpendicular to the median surface.

In such a PTC-resistor a surface-volume ratio of the ceramic base body can be achieved which provides a decreased ohmic resistance usually measured at a temperature of 25°C and which gives a characterization of the PTC component.

A structured PTC-resistor is thus described with a surface- volume ratio increasing the surface-volume ratio of bulk PTC- resistors as described above.

By increasing the area of a surface which is suitable for electrically connecting the base body of the PTC-resistor, the distribution of current flowing through the base body can be enhanced and the resistance of the component at 25°C (R25) reduced. A reduced resistivity at room temperature is beneficial for many applications of the PTC-resistor.

For example, in an overcurrent protection application the PTC-resistor is connected in series to circuitry to be prevented from overcurrent. Thus, the operating current which is required for the normal operation of the circuitry flows as a whole through the PTC-resistance . With low resistance at normal operation temperatures, voltage drop over the PTC- resistor can be minimized and thus power dissipation can be decreased.

In heating applications, a heating current flows through the PTC-resistor. According to Ohm's Law, the voltage required to provide a certain amount of heating current is lowered when the resistance of the PTC-resistor is lowered. This is beneficial in many applications where electrical voltage is limited, for example in automotive applications.

In an embodiment of the PTC-resistor, the at least one surface comprises bumps.

In another embodiment, the at least one surface of the base body comprises depressions.

In a preferred embodiment, the at least one surface comprises both, that is, bumps as well as depressions.

The shape of the at least one surface may be obtainable by holding a sheet with a predetermined thickness.

In another embodiment, not only the shape of one surface of the base body but the base body as a whole is, with respect to its shape, obtainable by folding a sheet.

The shape of the base body may thus be received by folding a sheet in a direction perpendicular to the median layer. In a preferred embodiment, a plurality of folds is carried out.

In another embodiment of the PTC-resistor, the at least one surface exhibits a plurality of folds, whereby each fold has a crest line running in parallel to the crest line of the adjacent fold.

Not only the at least one surface may exhibit a plurality of folds, but also the base body as a whole may be obtainable by applying a plurality of folds to a sheet.

Shaping the base body by folding the sheet may result in a base body which exhibits a wave-like shape.

In another embodiment of the PTC-resistor the ohmic resistance at room temperature decreases with an increase of the number of folds provided in the base body.

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The PTC-resistor may be produced by injection molding using a certain kind of feedstock.

The injection moldable feedstock preferably comprises a ceramic filler, a matrix for binding the filler and a content of preferably less than 10 ppm of metallic impurities.

The ceramic may for example be based on Bariumtitanate (BaTiθ3) , which is a ceramic of the perovskite-type (ABO3) .

For the injection molding process a feedstock could be used comprising a ceramic filler, a matrix for binding the filler and a content of less than 10 ppm of metallic impurities. One possible ceramic filler can be denoted by the structure:

Ba ₁ _ _x _ _y M _x D _y Ti ₁ _ _a _ _b N _a Mn _b O ₃

wherein the parameters are x = 0 to 0.5, y = 0 to 0.01, a = 0 to 0.01 and b = 0 to 0.01. In this structure M stands for a cation of the valency two, like for example Ca, Sr or Pb, D stands for a donor of the valency three or four, for example Y, La or rare earth elements, and N stands for a cation of the valency five or six, for example Nb or Sb. Thus, a high variety of ceramic materials can be used wherein the composition of the ceramic may be chosen in dependency of the required electrical features of the later sintered ceramic.

The ceramic filler of the feedstock is convertible to a PTC- ceramic with low resistivity and a steep slope of the resistance-temperature curve. The resistivity of a PTC- ceramic made of such a feedstock can comprise a range from 3 ωcm to 30000 ωcm at 25 ⁰ C in dependence of the composition of the ceramic filler and the conditions during sintering the feedstock. The characteristic temperature T _b at which the resistance begins to increase comprises a range of -30 ⁰ C to 340 °C. As higher amounts of impurities could impede the electrical features of the molded PTC-ceramic the content of

the metallic impurities in the feedstock is lower than 10 ppm.

The metallic impurities in the feedstock may comprise Fe, Al, Ni, Cr and W. Their content in the feedstock, in combination with one another or each respectively, is less than 10 ppm due to abrasion from tools employed during the preparation of the feedstock.

The preparation of the feedstock comprises using tools having such a low degree of abrasion that a feedstock comprising less than 10 ppm of impurities caused by said abrasion is obtained. Thus, preparation of injection moldable feedstocks with a low amount of abrasion caused metallic impurities is achieved without the loss of desired electrical features of the molded PTC-ceramic.

The tools used for preparation of the feedstock comprise coatings of a hard material. The coating may comprise any hard metal, such as, for example, tungsten carbide (WC) . Such a coating reduces the degree of abrasion of the tools when in contact with the mixture of ceramic filler and matrix and enables the preparation of a feedstock with a low amount of metallic impurities caused by said abrasion. Metallic impurities may be Fe, but also Al, Ni or Cr. When the tools are coated with a hard coating such as WC, impurities of W may be introduced into the feedstock. However, these impurities have a content of less than 50 ppm. It was found that in this concentration, they do not influence the desired electrical features of the sintered PTC-ceramic.

Where injection molding is used to form the mold, care must be taken regarding the metallic impurities in the mold to ensure that the efficiency of the PTC-ceramic is not reduced. The PTC-effect of ceramic materials comprises a change of the electric resistivity p as a function of the temperature T. While in a certain temperature range the change of the

resistivity p is small with a rise of the temperature T, starting at the so-called Curie-temperature T _Q the resistivity p rapidly increases with a rise of temperature. In this second temperature range, the temperature coefficient, which is the relative change of the resistivity at a given temperature, can have a value of 100%/K. If there is no rapid increase at the Curie-temperature the self regulating property of the mold is unsatisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the PTC-resistor will become apparent from the following detailed description in combination with the accompanying drawings .

Figure 1 is a perspective view of a PTC-resistor exhibiting a wave-like shape.

Figure 2 is a perspective view of a PTC-resistor where two different wave-like structures are crossing each other.

Figure 3 is a cross-sectional view of a PTC-resistor as shown in Figure 1.

DETAILED DESCRIPTION

Referring to Figure 1, a PTC-resistor is shown with a base body 1. The base body comprises a ceramic material with a positive temperature coefficient of the ohmic resistance. For example, the following material may be used as a ceramic material for example:

^(Ba 0,3290 ^Ca 0,0505 ^Sr 0,0969 ^Pb 0,1306 ^Y 0,005 ⁾

^(Ti 0,502 ^Mn 0, 0007 ^{) 0} I, 5045

A sintered body of this ceramic material has a characteristic reference temperature T] ₃ of 122 ⁰ C and - depending on the

conditions during sintering - a resistivity range from 40 to 200 ωcm.

The specific resistance p of that ceramic material lies in a range between 20 and 200 ωcm. In a preferred embodiment of the PTC-resistor, the base body comprises no further constituents influencing the ohmic resistance of the PTC- resistor as well as the temperature behavior of the ohmic resistance .

The base body of the PTC-resistor extends along a median layer 2 which means that the large dimensions denoted b and 1 of the base body 1 run parallel to the median layer 2 and the smaller dimension denoted h extends perpendicular to the median layer.

The base body is confined by surfaces, for example a top surface denoted 3 and a bottom surface denoted 4. In addition, further confining surfaces denoted 11 and 12 are provided.

At least one of the surfaces is configured to electrically contact the base body. In the example of Figure 1, the top surface 3 and a bottom surface 4 are configured to electrically connect the base body. This means that an electrical current which flows through the base body is distributed over the entire surface of the top surface 3 as well as over the entire surface of the bottom surface 4. This leads to a broad current distribution which helps to decrease the ohmic resistance of the PTC-resistor.

A configuration for the electrical contact may be achieved by coating the respective surfaces as illustrated in Figure 3. With reference to Figure 3, conductive layers 31 and 41 are provided on the top and bottom of the base body 1, respectively. These conductive layers may be applied by

screen printing of a paste containing metal particles or by coating techniques such as sputtering or vacuum deposition.

It may turn out to be convenient to further apply external terminals to the conductive layers 31 and 41, for example leads which may be chosen to be contact wires. The contact wires may be attached to the conductive layers by soldering or welding.

In Figure 1 two different surfaces are shown which have an area larger than the area of the parallel projection of the base body. When speaking in terms of parallel projection, a projection perpendicular to the median surface is meant. Such a projection is illustrated in Figure 3 by means of parallel light illustrated by the arrows falling in a perpendicular direction to the median layer 2 on top of the base body 1. The projection results in a shade on the projection layer 6 running in parallel to the median layer 2. The outline of the shadow 61 limits an area which is smaller than the area of the top and bottom surface 3, 4.

According to an embodiment of the PTC-resistor at least one surface comprises one or more bumps. In the example of Figure 1, bumps 71, 72, 73 are provided on the top surface 3 of the base body 1.

In another embodiment the PTC-resistor has at least one surface which comprises depressions. In the example of Figure 1, depressions 81 and 82 are provided on the top surface 3. Further depressions 712, 722, 732 are provided on the bottom surface 4.

The shape of the top surface 3 and the bottom surface 4 may be achieved by folding a sheet with a predetermined thickness. By forming the top surface and the bottom surface in the described manner, a base body 1 is obtained having a shape which may be achieved by folding a sheet.

In the example of Figure 1 only the shape, not the manufacturing process, may be regarded as the outcome of a process where a sheet 5 has been folded such that folds 91 and 92 extend in a perpendicular direction to the median layer 2. Folding the sheet uniformly results in folds being separated from one another by a constant distance. Further, by a suitable folding process folds can be achieved which exhibit crest lines 711, 721, 731 which denote the top of each fold and which run parallel to one another.

While the shape of the base body shown in Figure 1 may be achieved by folding a layer with a predetermined thickness, the manufacturing of a PTC-resistor as shown in Figure 1 cannot be achieved by a method using injection molding of a PTC-ceramic

Further, the PTC-resistor shown in Figure 1 exhibits a wave- like shape which, in particular, becomes apparent from Figure 3 showing a cross-section of the base body of Figure 1.

The preparation of folds 91, 92 results in a shape of the base body 1 where for each bump 71, 72, 73 on the top surface 3, a corresponding depression 712, 722, 732 is provided on the opposite side, namely the bottom surface 4 of the base body.

In order to facilitate electrical contacting of the base body 1, conductive layers 31 and 41 are provided on the top surface 3 and the bottom surface 4 respectively as explained in Figure 3.

Now turning to the geometrical dimensions of the base body shown in Figure 1, the length of the base body is denoted 1, the width of the base body is denoted b, the thickness of the layer being folded is denoted d and the height of the base body 1 is denoted h.

In general, a wide variety of different values for 1, b, d, h may be chosen depending on the concrete application for the PTC-resistor.

The height of the base body is twice the thickness d of the layer plus 0.5 mm.

An example for the effect of folding a layer in order to achieve a decreased ohmic resistance for a PTC-resistor is shown in the table below.

In the first column the different types of components are given. All types are shaped according to the example of Figure 1.

Here, S denotes the number of segments, whereby each segment runs from the top of a first projection on the one surface to the top of the adjacent projection on the opposite surface. An example for a segment as it is used in Table 1 is given in Figure 3, denoted with reference numeral 100. In Table 1, h stands for the height of the base body 1 and is given in milimeters .

The dimensions D are also given in the Table, whereby D stands for the product of the length of the base body 1 (first item) and the width b of the base body (second item) .

The respective size of the projection of the shadow area denoted P is given in mm^ . The ohmic resistance R measured at a temperature of 25°C is given in the Table in ω. In the next column of the Table the ratio of two areas is given. The first area Al is the size of the shadow area of the type mentioned in the Table. The second area Al is the area of a disc shaped resistor having the same mass as the waved resistor and at the same time having a thickness according to the respective value for h. Ratio is calculated as A2/A1.

In the last column, a minimum value for the maximum switching current in case of application of the resistor as a switch is also provided and is given in A.

The second area is the area of a PTC-resistor having the same mass of ceramic material but having a flat shape and having the thickness as given in the third column of the Table.

As can be seen from Table 1, the shadow area of the different embodiments is always smaller than the area of a disc-shaped PTC-resistor.

All different types mentioned in the first column have a maximum voltage that can be applied of 265 V and a breakdown voltage of more or equal to 420 V.

Now turning to the example of Figure 2, the number of bumps and depressions is increased with regard to the example of Figure 1. The base body 1 in Figure 2 has the shape of two waves, each wave comprising several folds. The first wave comprises the folds 91 and 92. The folds run in the same direction and exhibit the respective parallel crest lines 711, 721.

Another wave is shaped in the base body 1 outlining the folds 93 and 94. These folds also run in the same direction. The first group of folds 91, 92 runs in a perpendicular direction

to the second group of folds 93, 94. Thus, Figure 2 exhibits a kind of cross-over wave structure for the PTC-component .

The folds 91, 92, 93, 94 result in respective bumps on the top surface 3 of the base body 1 denoted 71, 72, 73, 74.

Between the bumps 71, 72, 73, 74 depressions 81, 82, 83, 84, 85 are formed.

Due to the increased complexity of the surface with regard to the embodiment of Figure 1, the surface to shadow area ratio is increased in the embodiment of Figure 2.

What can be derived from Table 1 is that independent of the specific resistance of the PTC-ceramic, the ohmic resistance of the PTC-component can be decreased by increasing the number of segments used, see for example column 6 compared to column 2.