FR2508436A1 - Capacitor using ceramic dielectric mixt. contg. barium titanate - having small capacitive variation over wide temp. range - Google Patents

Abstract

Ceramic dielectric mixt. contains 80-94% by wt. barium titanate, 1-3% neodymium oxide, 0-4% bismuth oxide, 2-5% bismuth zirconate, and 2-8% sintered glass so that the whole is 100%. Pref. forms include, the proportion of bismuth oxide is 1-4%, the barium titanate has a molar relationship of TiO2/BaO between 1.06 and 1.09. The sintered glass includes more than 70% bismuth oxide and is pref. of the composition 81% bismuth oxide, 17% lead oxide and 2% boron oxide. An alternate mixt. includes 0-2% pref. 0.5% of an adjuvant chosen from the oxides of zinc, manganese or their mixtures. The patent also includes the capacitor formed from at least one layer of dielectric and two electrodes pref. several electrodes of an 80% silver, 20% palladium composition. The ceramic mixt. has a small variation in capacity over a large range of temps. in the presence of an exterior electric field notably for a temp. range of -55 deg.C to 125 deg.C. the variation is less than or equal to +15% for a signal of 1 V. at a freq. of 1 KHz and the variation is between +15% and -25% in an exterior field of 50V.

Description

 The present invention relates to a dielectric ceramic composition based on barium titanate and a capacitor using this composition. It relates more particularly to a ceramic composition having a small relative variation of capacity in a very broad temperature range allowing it to reach the BX class.

 The article of the journal FERROELECTRICS entitled "Electrode and materials problems in ceramic capacitors" by WR BUESSEM and TI PROKOROWICZ published in the volume 10 - pages 225 to 230 1976 discloses compositions based on barium titanate at low sintering temperature using silverpalladium type electrodes much less expensive than gold or palladium electrodes. In particular, Figure 6 of this article shows results of measuring the relative capacitance versus temperature variation for a constant dielectric of 1450.

The voltage variation curve of 19.7 KV / cm or 2 volts / micron corresponds well to class BX, that is to say that the capacitance variation of said dielectric used in baked sheet of 25 u at 50 volts ( rated voltage) responds well to this class of product.

However, the article merely states that this composition is based on barium titanate, a glass and unspecified additives. US Pat. Nos. 3,619,220, 3,682,766 and 3,811,937 to which the article appears to refer with respect to these compositions show that these compositions are based on glass comprising essentially cadmium oxide (approximately 33%) as well as bismuth oxide and lead oxide (25% by weight for each). Among the possible additives, it seems that the neodymium and lanthanum oxides may be suitable.

However, as is clearly shown in the article cited above, these compositions do not all respond to the class
BX and those who respond to it are not described. In addition, the latter have a relatively low dielectric constant.

The compositions according to the invention do not have this disadvantage while having an excellent temperature resistance in
Absence and in the presence of an external electric field allowing them to reach class BX. (This class of components is defined in particular by a relative variation of capacitance C / C between -55 ° C and + 125 ° C less than or equal to + 15% for a 1 V signal at the frequency of 1 KHz and a variation relative capacity of less than + 15% and -25% in the presence of an external DC field of the order of 50 V. The precise definitions can be found in the American standard MIL C 11015 D).

 For this purpose, the compositions according to the invention are characterized in that they comprise a mixture comprising from 80 to 94% by weight of barium titanate, from 1 to 3% by weight of neodymium oxide, from 0 to 4 % by weight of bismuth oxide, from 2 to 5% by weight of bismuth zirconate and from 2 to 8% by weight of glass frit.

 According to a preferred embodiment, the compositions according to the invention are characterized in that they comprise from 1 to 4% by weight of bismuth oxide.

 As will be seen later, it has indeed been found that such compositions (known to those skilled in the art as Type II compositions) not only had a sintering temperature lowered compared to usual temperatures sintering compositions based on barium titanate, but that said compositions also surprisingly possessed excellent electrical properties in a very wide temperature range.

 Preferably, non-stoichiometric barium titanate having a TiO 2 / BaO molar ratio between 1.06 and 1.09 will be used.

 The glass frits that can be used in the context of the present invention may have different compositions. These glass frits are made as is well known to those skilled in the art, by mixing the various ingredients used to make the glass, and then melting them at high temperature, cooling to room temperature to obtain a glass which is then ground into very fine particles, this powder being called glass frit. It has been found that it is necessary to use glass frits having at least 70% by weight of bismuth oxide, the other components being lead oxide and / or boron oxide. A composition which was most satisfactory included substantially 81% by weight of bismuth oxide, 17% by weight of lead oxide and 2% by weight of boron oxide.

 In order to further improve the electrical properties and / or lower the sintering temperature of these compositions, up to 2% by weight of the usual adjuvants well known to those skilled in the art can be added to achieve this purpose. In particular, it is possible to add zinc oxide and / or manganese oxide, as well as their various mixtures, in a proportion by weight preferably of about 0.5%.

 It should be noted that the compositions according to the invention make it possible to produce multilayer ceramic capacitors comprising electrodes having more than 70% by weight of silver and even 80% and more by weight of silver. As will be seen later, the compositions of the invention have excellent resistance in a high electric field, of the order of 3 V / micron for dielectric constants of the order of 2000.

The invention will be better understood with the aid of the following exemplary embodiments, given in a non-limiting manner, together with the figures which represent:
FIG. 1, a relative variation curve of the capacitance of the capacitor of example 1,
FIG. 2, a relative variation curve of the capacity of the capacitor of example 2,
FIG. 3, a relative variation curve of the capacitance of the capacitor of example 3.

EXAMPLE 1
A porcelain jar containing 7 kg of zirconia beads, 2 liters of solvent, 200 g of polymethyl methacrylate and 2 kg of powder having the composition below is mixed in:
Barium titanate stoichiometry 1.08
Ba Ti O3 89.1%
Neodymium oxide Nd2 03 1.9%
Zinc oxide Zn O 0.25%
Manganese oxide Mn O 0.25%
Bismuth zirconate 2 Bi2 3 3 Zr 2 3.8%
Glass frit (consisting of 81% by weight of
Bi 2 O 3, 17% Pb O, 2% B2 03) 4.7% (all percentages given above are percentages by weight).

 All of these constituents are mixed for about 15 hours. The slip obtained is cast on a rigid and smooth support, and then dried.

With the aid of this ceramic sheet, multi-layer capacitors consisting of an alternating stack of ceramic dielectric layers and metal electrodes are produced. These are deposited on the dielectrics according to the well-known technique of screen printing. They consist of a palladium-silver alloy comprising about 80% by weight of silver. The capacitors thus produced comprised 20 metallized layers having reinforcing functions and interconnected respectively the even-rank layers and the odd-rank layers. These capacitors are then sintered at a temperature of 11300 Celsius in air. Each ceramic layer has a thickness of about 30 microns. The results obtained are shown in Table I below:
Capacity 50 nF
Tangent s xi0 + 4 (under tension
1 volt at 1 KHz) 230
Insulation resistance under 63 volts 50 G Ohms
Relative variation of capacity
AC / C between 0 and 1 volt per micron - 4%
Temperature class (MIL C 11 015 D) BX
Dielectric constant at 20 Celsius 2100
Coefficient of aging at 20 Celsius - 0.8% per
decade
350 volts breakdown voltage
TABLE I
In Table I above, it can be seen that the dielectric according to the invention as well as the capacitors made from this dielectric have good properties and in particular they correspond to a temperature class BX. Without going into the detail of above mentioned standard, the class BX indicates in particular that between - 550C and + 125 C, the relative capacitance variation under an effective volt at a frequency of 1 KHz is less than or equal to t 15% in the absence of outside field of polarization, and that the relative variation of capacity under continuous nominal voltage (63 volts in the case above) is less than + 15% and -25% still in the same temperature range.

 Detailed results of measurement of relative variation in capacitance \ C / C are given in Figure 1 as a function of temperature. On the abscissa, the temperatures of - 550C and + 1500C are plotted, while on the ordinates the relative variations of the EC / C capacity in percentage are plotted. The curve NO 1 shows the relative capacitance variations of the capacitors of Example 1 under an effective volt at 1 KHz. The nominal value of the capacitance was taken at 25 ° C, which corresponds to this temperature at tC / C equal to 0. It is noted on this curve a maximum of the relative variation of capacity around 1300C, this one n However, the relative variation of capacity tends regularly towards - 8% value reached for - 559C.

 Curve N "2 represents the relative variation of capacitance 8 C / C in the presence of an external continuous field of 63 Volts, measured using a signal of 1 Volt having a frequency of 1 KHz.

Here again, the results obtained are particularly excellent.

This composition meets the BX standard, the dotted limits of which are indicated in the figure.

 FIG. 1 also shows the curve N "3 which represents the variation of the tangent of the angle of loss, tangent as a function of the temperature, the ordinates for interpreting this curve are read on the right. This curve shows the general trend of variation of this angle of loss.

So, as we can see, these measures are particularly good.

EXAMPLE 2
A mixture containing also 2 kg of powder, the proportions of which are given below, is prepared under the same conditions as those indicated in Example 1:
Barium Titanate Ba Ti O3 87.35%
Neodymium oxide 1.86%
0.23% manganese oxide
0.23% zinc oxide
Bismuth zirconate 3,72in
Bi2 bismuth oxide 3 1.96%
Glass frit 4.65%
As can be seen, the composition of the powder in this example is substantially identical to the difference that bismuth oxide has been added. Multilayer ceramic capacitors are made as in Example 1 in the same conditions, and the results obtained are collated in Table II below (thickness of the dielectric after sintering 26 u):
Capacity 51 nF
Tangent 6 xl0-4 (under a volt at 1 KHz) 180
Insulation resistance under 63 volts 200 G Ohms
Relative capacity variation of -2%
0 and 1 volt per micron
Temperature class (MIL C 11 015 D) BX
Dielectric constant at 200 Celsius 1850
Coefficient of aging at - 200 Celsius - 0.6% per
decade
Breakdown voltage greater than or equal to 400 volts
TABLE II
As can be seen, the fact of adding in the composition of the bismuth oxide makes it possible to substantially modify certain properties of the dielectric according to the invention. In particular, it is found that the insulation resistance at nominal voltage is very much improved since it is multiplied by 4 and the relative capacity variation is further improved.

 This improvement in the results is very clearly confirmed by the relative capacitance variation curves as a function of the temperature shown in FIG. 2. In this FIG. 2, the abscissa and ordinate notation and the significance of the different curves are the same as in the corresponding case of example 1. In this FIG. 2, the NO I curve of relative capacitance variation under an effective volt at 1 KHz is particularly excellent since it is substantially in the form of a straight line inclined by report horizontally. In particular, unlike in the case of Figure 1 corresponding, there is no hump or rapid variation of d C / C.

 The curve NO 2 of FIG. 2, which corresponds to the curve N "2 of FIG. 1, also shows a very marked improvement in the nominal voltage of bC / C. As in the previous case, no sudden variation of the parameter is observed. measured as a function of temperature, curve No. 2 being substantially flat.

 Curve N "3, which corresponds to curve N" 3 of FIG. 1, also makes it possible to observe that the tangent of the loss angle, tangent S, has a much smaller variation in this case than in the case of Example 1

 Therefore, it is clear from these figures that the incorporation of bismuth oxide makes it possible to obtain a significantly improved dielectric with respect to the dielectric of Example 1.

EXAMPLE 3
Dielectric compositions and capacitors are produced exactly under the same conditions as in Example 2 above, however using palladium-silver electrodes comprising 80% silver and 20% palladium and sintering at 1100.degree. all things being equal.The results obtained are given in Table III below:
Capacity 53 nF
Tangent S x 104 (under a volt at 1 KHz) 180
Insulation resistance under 63 volts 500 G Ohms
Relative variation of capacity between
0 and 1 volt per micron - 3%
Temperature class (MIL C 11 015 D) BX
Consistency at 200 Celsius 1940
Coefficient of aging at 20 Celsius - 1% per
decade
(thickness of each dielectric layer: 26 u)
TABLE III
Although the silver concentration of the electrodes has been increased, a product having excellent characteristics is obtained. The parameters measured are substantially identical to those of Example 2 shown in Table III. However, it can be considered that this embodiment is still better than the previous one since the sintering temperature has been lowered by about 30, which represents a significant gain in energy, while the insulation resistance increases correlatively.

 On the capacitors made in accordance with this example, the same measurements were made as in the case of Examples 1 and 2, measurements shown in FIG. 3, in which the curves 1, 2 and 3 have the same meaning as in the previous figures. .

 It can be seen in this FIG. 3 that the results obtained for the curve N "1 are much better than in the case of FIG. 1, although having a hump around 1350, this curve N" 1 is also generally better than the curve NO 1 of Figure 2, the relative capacity variation being lower.

 The curve N "2 of Figure 3 is also significantly improved with respect to the corresponding curve of Figure 1. It is substantially straight and substantially horizontal which is also an improvement over the curve N" 2 of Figure 2.

 The curve N "3 which represents the variation of the tangent of the angle of loss is also better than in the case of FIG. 3 and in the case of FIG. 1 but slightly less good than in the case of FIG. especially at low temperatures.

EXAMPLE 4
Using multi-layer capacitors made according to Example 2, an aging test is carried out under the following conditions: the capacitors are subjected for 1000 hours at a temperature of 1250 ° C. and at a continuous voltage of 125 volts, that is to say substantially twice the nominal or service voltage of these capacitors.

The results obtained are shown in Table IV below:

<tb><SEP> BEFORE <SEP> TEST <SEP> AFTER <SEP> TEST <SEP> VARIATION
<tb> Average <SEP> value <SEP> of
<tb><SEP> the <SEP> capacity <SEP> (nF) <SEP> 51.1 <SEP> 50.3 <SEP> - <SEP> 1.6% <SEP>
<tb> Average <SEP> value <SEP> of
<tb><SEP><SEP> resistance <SEP> 287 <SEP> 257 <SEP> - <SEP> 10% <SEP>
<tb><SEP> Isolation <SEP> (GL) <SEP>
<Tb>
TABLE IV
As can be seen from this table, the average value of the capacity of the parts undergoes an extremely low relative variation during this aging test as well as the insulation resistance of said capacitors.

 The above examples and the corresponding curves thus clearly show the unexpected and exceptional results of the ceramic compositions according to the invention, in particular when said compositions are used for the production of multi-layer ceramic capacitors.

Claims (12)

 1. Dielectric ceramic composition, characterized in that it consists of a mixture comprising from 80% to 94% by weight of barium titanate, from 1 to 3% by weight of neodymium oxide, from 0 to 4% by weight of bismuth oxide, from 2 to 5% by weight of bismuth zirconate and from 2 to 8% by weight of glass frit, the combination being selected to make 100% by weight.  2. Ceramic dielectric composition according to claim 1, characterized in that it comprises from 1 to 4% by weight of bismuth oxide.  3. Ceramic dielectric composition according to one of claims 1 or 2, characterized in that the barium titanate has a molar ratio Ti O2 / Ba O between 1.06 and 1.09.  4. Ceramic dielectric composition according to one of claims 1 to 3, characterized in that the glass frit comprises more than 70% by weight of bismuth oxide.  5. Ceramic dielectric composition according to claim 4, characterized in that it comprises substantially 81% by weight of bismuth oxide, 17% by weight of lead oxide, and 2% by weight of boron oxide.  6. ceramic dielectric composition according to one of claims 1 to 5, characterized in that it further comprises from 0 to 2% by weight of adjuvant.  7. Ceramic dielectric composition according to claim 6, characterized in that it comprises about 0.5% by weight of adjuvant.  8. Ceramic dielectric composition according to one of claims 6 or 7, characterized in that the adjuvants are selected from oxides of zinc, manganese and their mixture.  9. ceramic dielectric composition according to claim 8, characterized in that it comprises about 0.25% by weight of zinc oxide and 0.25% by weight of manganese oxide.  10. Electrical capacitor comprising at least one dielectric layer and at least two electrodes, characterized in that the dielectric layer is according to one of claims I to 9.  11. An electric capacitor according to claim 10, comprising a plurality of metal electrodes separated by dielectric layers, the even-numbered and odd-rank metallic electrodes being respectively connected to one another, characterized in that said electrodes consist of a mixture of of palladium and silver comprising at least 70% by weight of silver.  12. Electrical capacitor according to claim 11, characterized in that the electrodes comprise 80% by weight of silver and 20% by weight of palladium.