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Dokumentenidentifikation EP0759626 03.04.1997
EP-Veröffentlichungsnummer 0759626
Titel Keramische dielektrische Zusammensetzungen mit niedriger Brenntemperatur und damit hergestellte Kondensatoren
Anmelder Ferro Corp., Cleveland, Ohio, US
Erfinder Wilson, James M., Victor, New York 14564, US;
Symes, Walter J., Jr., Dundee, New York 14837, US
Vertreter derzeit kein Vertreter bestellt
Vertragsstaaten AT, DE, FR, GB, NL
Sprache des Dokument En
EP-Anmeldetag 19.08.1996
EP-Aktenzeichen 963060280
EP-Offenlegungsdatum 26.02.1997
Veröffentlichungstag im Patentblatt 03.04.1997
IPC-Hauptklasse H01G 4/12

Beschreibung[en]

This invention relates to low fire ceramic dielectric compositions and to multi-layer ceramic capacitors made therefrom, and is particularly (but not exclusively) concerned with dielectric ceramic compositions for the X7R variety suitable for producing low fire multi-layer ceramic capacitors which may have dielectric constants in excess of 3200.

There are a lot of U.S. patents which disclose various ceramic dielectric compositions that exhibit X7R characteristics, i.e. compositions which meet the U.S. Electronics Industries Association (EIA) standard requiring a temperature coefficient of dielectric constant (K') that varies no more than about 15% from the reference value at 25°C over a temperature range of -55°C to 125°C. Much of this wealth of prior art can be attributed to the fact that even small changes in the constituents and/or ratios of the constituents in dielectric compositions may generate new and unexpected characteristics. These new characteristics may produce enhanced properties when used in a multi-layer ceramic capacitor.

For example, U.S. patent no. 4,816,430 discloses a composition having X7R characteristics, but it is necessary to fire products produced from such compositions at relatively high temperatures, i.e. temperatures in excess of 1280°C. As a consequence, capacitors made from those compositions must utilize expensive electrode materials such as, for example, pure palladium. Our U.S. patent no. 5,128,289 discloses dielectric ceramic materials having X7R characteristics, but they also constitute high fire compositions which must be sintered at temperatures which also must be in excess of 1280°C. These latter compositions, however, have the advantage that they produce products having very dense, fine grained ceramic microstructures, thus permitting the use of much thinner dielectric layers during the production of multi-layer capacitors. Such capacitors in turn exhibit higher dielectric constants (K'), and thus permit the use of less expensive electrode materials for a given capacitance value.

U.S. patents nos. 4,540,676 and 5,296,426 disclose so-called low fire dielectric compositions having X7R characteristics, but although it is possible to fire capacitors made from such compositions in the range of 1100 to 1150°C, such capacitors do not necessarily exhibit dielectric constant values in excess of 3200.

We have now devised a low fired ceramic dielectric composition in powder form which has X7R characteristics, and which is especially useful for making low fire multi-layer ceramic capacitors.

According to the present invention, there is provided a low fire ceramic dielectric composition in powder form capable of producing a multi-layer capacitor having a dielectric constant which does not vary more than 15% from its value at 25°C over a temperature range of -55°C. to 125°C., which composition comprises a barium titanate and neodymium oxide mixture forming a host material present in the range of 88.6 to 91 wt.% of the composition, the barium titanate constituting from 97.89 to 98.19 wt.% of the host material, and the neodymium oxide constituting from 1.81 to 2.11 wt.% of the host material; a Bi2O3.2TiO2 sintering aid present in the range of 7.98 to 10.0 wt.% of the composition; a glass powder present in the range of 0.8 to 1.6 wt.% of the composition; and optionally a trace amount of manganese dioxide.

The invention also includes a low fire multi-layer capacitor made from a composition of the invention.

The figures given are approximate only.

It is possible, according to the preferred aspects of the invention, to produce capacitors that are capable of developing dielectric constants (K') in excess of 3200. Among the preferred multi-layer ceramic capacitors of the invention are those capable of generating dissipation factors which are equal to or less than 2.5% at 1 KHz test frequency with a signal amplitude of 1.0 VRMS.

The compositions of the invention can be free of manganese dioxide or they may contain a trace amount, for example up to about 0.1%, more usually up to about 0.075% by weight. The MnO2 can be provided, for example, by including a solution of manganese nitrate in the mixture before firing to make a composition of the invention. Preferably, up to about 0.305 wt.% of a 50% aqueous solution (or the equivalent amount of a different concentration solution) is used.

In the compositions of the invention, the barium titanate is preferably calcined at a temperature in the range of 1100°C to 1225°C before being mixed with the neodymium oxide. Preferably, the host material is calcined at a temperature in the range 1200°C to 1365°C. The Bi2O.2TiO2 is preferably present in the range 9.983 to 7.983 wt.% of the composition, and the host material is preferably present in the range 89.013 to 91.013 wt.%.

In the capacitors of the invention, the dielectric constant is preferably equal to or greater than 3000. Preferably, the capacitors are fired at a temperature of up to 1100°C.

In one preferred procedure of the invention, high purity barium titanate of uniform grain size and neodymium oxide are milled together in an aqueous slurry, dried and reacted at an elevated temperature. This reacted powder is then reduced in particle size by mechanical pulverisation after which Bi2O3.2TiO2 and a powdered low melting point glass are added which are milled together in an aqueous slurry, dried and pulverised to form a finely divided, homogeneous powder of the invention. This can then be made into a capacitor of the invention by, for example, mixing the powder with a solvent based PVB (polyvinyl butyral) binder and casting it into sheets which are then screen printed with electrode material and laminated to form multi-layer capacitors. Such preferred capacitors have exhibited dielectric constants of 3500.

The ceramic dielectric compositions of the invention are preferably formulated from a host material comprising a mixture of high purity, uniform particle size BaTiO3 and Nd2O3. The BaTiO3 used in the samples referred to hereinafter was calcined independently of the Nd2O3, and after being combined with Nd2O3, the mixture was calcined and then mixed with finely powdered Bi2O3.2TiO2 as a sintering aid and finally powdered low melting point glass. For the above-noted mixture, the high purity barium titanate was of the type which is sold by the Transelco, Division of Ferro Corporation as product code 219-9, and which is produced via a precipitation process similar in nature to that taught for example in our U.S. patent no. 4,670,243.

By way of example, each host material of the sample dielectric compositions listed in the following Tables was milled in an aqueous slurry in a vibratory type sweco mill, using ZrO2 grinding media, for 1.5 hours. This slurry was dried and the dried slurry was passed though a 10 mesh (No. 9 Tyler) screen (2.0 mm opening) prior to calcining. This calcined material was then pulverised to a fine powder and mixed with Bi2O3.TiO2, glass powder and Mn(NO3)2, in the form of a 50% aqueous solution, at the listed percentages. The resulting compositions were milled in an aqueous slurry in a rolling ball mill with Al2O3 grinding media for four hours, dried and pulverised to a finely divided homogenous dielectric powder of the invention.

By way of further example, in order to make capacitors of the invention, the dielectric powders were mixed with a commercially available solvent based PVB binder system to prepare a slip for tape casting. The resultant slip was cast into sheets, cut to size and printed with a 70%Ag 30%Pd internal electrode paste. These sheets were then stacked, laminated and cut into individual multi-layer ceramic capacitors having a conventional configuration, such as illustrated for example in Fig. 2 of our U.S. patent no. 4,379,319, to which reference should be made for further details. The capacitors were then fired over a range of temperatures from 1070° to 1100°C. The fired capacitors were silver terminated with the terminations being fired on 750°C for fifteen minutes, and were tested to determine certain electrical properties thereof as noted hereinafter.

The following chart lists the compositions of four different host materials which were used in producing certain of the sample dielectric compositions referred to hereinafter. Host Material wt.% Nd2O3 wt.%BaTiO3 mole% Nd2O3 mole%BaTiO3 No. 1 1.81 98.19 1.262 98.738 No. 2 1.91 98.09 1.332 98.668 No. 3 2.01 97.99 1.402 98.598 No.4 2.11 97.89 1.472 98.528

Each of the above host mixtures was calcined in a laboratory box kiln at 1350°C for three hours. Four different low firing dielectric compositions 1A, 2A, 3A and 4A were then formulated from these hosts No. 1, No.2, No. 3 and No.4, respectively, using the following formula. SAMPLE COMPOSITION FORMULA COMPONENT wt.% Host Material 91.013 Bi2O3.2TiO2 7.983 glass powder 0.799 Mn(NO3)2 at 50% 0.205

Electrical properties of multi layer capacitors made from samples 1A-4A are listed hereinafter in Table I. Electrical Properties Material 1A 2A 3A 4A Firing temp°C (a) 1080 1100 1080 1100 1080 1100 1080 1100 diel.thk. mils(b) 0.77 0.77 0.91 1.09 0.80 0.86 0.82 0.82 diel. K' (c) 2850 2842 2962 2987 2675 2740 2805 2812 % DF (d) 2.26 2.27 2.09 2.07 2.14 2.18 1.99 2.03 -55°%Δ (e) -14 -12 -7 -11 -10 -10 -7 -9 85°%Δ (e) -4 -2 -3 -1 -5 -2 -6 -3 125°%Δ(e) -1 -1 +2 +9 -2 0 -5 0 25°R.C(f) >10K >10K >10K >10K >10K >10K >10K >10K 125°R.C(f) 704 1500 3530 3283 749 415 3550 2927
Notes to Table

(a) temperature in °C. at which capacitors are fired.

(b) thickness of active dielectric layers in mils

(c) dielectric constant.

(d) dissipation or loss factor in %.

(e) %Δ means percent change in capacitance as measured at given temperature.

(f) R.C means resistance capacitance product generated by measuring the resistance of a capacitor and multiplying this value by the capacitance of the capacitor, the units being meg-ohm/micro-farads.

Referring to the results listed in Table I, it can be seen that all four sample compositions meet the E.I.A. specification for X7R type dielectrics; and the sample compositions 2A, which is made from the No.2 host material, exhibits the highest dielectric constant. Therefore, to determine if the calcining temperature of the BaTiO3, prior to its being combined with Nd2O3, could have any significant effect on the sample compositions, samples of the BaTiO3 powder were calcined at various different temperatures before being mixed with Nd2O3 in the proportions corresponding to the above-noted host No.2. The precalcined BaTiO3 and Nd2O3 were then mixed, calcined, and mixed per the foregoing sample formulation with the other components for forming the additional sample compositions referred to hereinafter as samples 5A, 6A, 7A, 8A and 9A. Specifically, samples 5A, 6A, 7A, 8A and 9A were produced using host material No. 2 in which the BaTiO3 was precalcined at 900°C., 1000°C., 1100°C., 1200°C and 1225°C., respectively. (A sample using BaTiO3 calcined at 1177°C. was also tested but has already been reported on as sample 2A.) The electrical properties of samples 5A-9A are listed in the following Table II. Electrical Properties Material 5A 6A 7A 8A 9A Firing temp°C 1080 1100 1080 1100 1080 1100 1080 1100 1080 1100 diel.thk. mils 0.92 0.94 1.06 1.02 0.93 0.94 0.94 1.01 0.94 1.02 diel.const. 2481 2537 2437 2507 2858 3110 2794 2889 2939 2932 % DF 1.92 1.53 1.73 1.51 1.90 2.00 1.05 2.02 2.11 2.10 -55°%Δ -20 -17 -20 -16 -9 -10 -9 -10 -9 -10 85°%Δ -8 -6 -8 -4 -6 -4 -4 -2 -4 -1 125°%Δ -18 -13 -16 -9 -6 -1 +4 +3 +5 -3 25°R.C 1504 1229 467 658 >10K >10K >10K >10K >10K >10K 125°R.C 1457 1152 2188 1489 300 360 2600 2100 3600 2450

Referring to the results listed in Table II, it can be seen that when the BaTiO3 is calcined at equal to or less than 1000°C. the dielectric formulation generated does not meet E.I.A. X7R specifications. Comparing the remaining three calcines, it can be seen that composition 7A develops the highest dielectric constant at 1100°C., but composition 2A (Table I) generates nearly as high a dielectric constant along with a tighter T.C.C. as well as a much higher R.C product at 125°C.

For another series of tests relating to calcining temperatures, five samples of the host material No. 2, each of which included BaTiO3 particles precalcined at 1177°C., were employed in tests. The first sample host material was not calcined, and the remaining four samples were calcined at temperatures of 1200°C., 1250°C., 1300°C. and 1365°C., respectively. The five samples were then mixed per the above sample composition formula with the remaining components to produce the compositions referred to hereinafter as samples 10A, 11A, 12A, 13A and 14A, respectively. (A sample using host No. 2 calcined at 1350°C. has already been reported on as sample 2A.) The electrical properties of samples 10A-14A are listed in the following Table III. Electrical Properties Material 10A 11A 12A 13A 14A Firing temp°C 1080 1100 1080 1100 1080 1100 1080 1100 1080 1100 diel.thk. mils 1.00 0.99 0.90 0.92 0.88 0.89 0.92 0.96 1.01 0.93 diel.const. 2180 2188 2575 2598 2541 2539 2539 2608 3500 3430 % DF 1.92 1.87 2.12 2.15 2.22 2.22 2.08 2.17 1.93 1.98 -55°%Δ -17 -14 -14 -13 -15 -14 -15 -14 -6 -8 85°%Δ +1 +2 0 +1 +1 +2 -2 0 -6 -5 125°%Δ +7 +10 +8 +11 +10 +2 +2 +6 -7 -2 25°R.C >10K >10K >10K >10K >10K >10K >10K >10K >10K >10K 125°R.C 9000 6000 5000 2700 4000 2500 2500 1800 820 1290

It can be seen from Table III that increasing the temperature at which the host is calcined generates a corresponding increase in dielectric constant. Although increasing the calcining temperature of the host material may increase the dielectric constant even more, the drop in insulation resistance at 1365°C. indicates there may be continued loss at higher temperatures. Any further increase in temperature would also necessitate the use of more expensive, higher temperature capacity calcining saggers.

Tests were also conducted on three sample compositions 15A, 16A and 17A in each of which, except for sample 17A, the ratio of the host material No. 2 to the Bi2O3.2TiO2 differed from the proportions (wt.%) listed in the foregoing sample composition formula. The proportions of these components, except for the glass powder and the Mn(NO3)2 which remain in the same proportions, are listed in the following chart, and the electrical properties thereof are listed in Table IV. Sample wt.% Host wt.% Bi2O3.2TiO2 15A 89.013 9.983 16A 90.013 8.983 17A 91.083 7.983
Electrical Properties Material 15A 16A 17A Firing temp°C 1080 1100 1080 1100 1080 1100 diel.thk. mils 0.85 0.88 0.92 0.95 0.82 0.84 diel. constant. 3068 333 2968 3176 3179 3328 % DF 1.70 1.66 1.82 2.03 1.98 2.17 -55°%Δ -5 -6 -7 -8 -7 -8 85°%Δ -8 -10 -8 -10 -8 -10 125°%Δ -6 -11 -5 -10 -6 -10 25°R.C >10K >10K >10K >10K >10K >10K 125°R.C 1011 152 2366 411 2124 318

The previously listed data illustrate a relationship between the level of Bi2O3.2TiO3 present in the low fire and the %d.f. of the resultant multi-layer ceramic capacitor. Increasing the amount of Bi2O3.2TiO2 improves the d.f. value which will in turn make the material more useful for thinner dielectric layer capacitors. This is due to the fact that present E.I.A. specifications state that X7R type dielectrics be electrically evaluated using a 1VRMS signal. If the test signal is kept at a constant amplitude, the d.f. value of a given material will increase as dielectric thickness deceases. In contrast to this, as the dielectric thickness of a capacitor decreases the capacitance of said capacitor increases. Present and future trends towards miniaturization dictate the need for dielectric materials which are able to produce d.f. values equal to or less than 2.5% @ approximately 10µm dielectric thickness.

In addition to the previously reported sample composition 15A, the glass powder levels of two additional samples 18A and 19A were altered from the amounts listed in the first-noted sample formulation, as shown by the proportions listed in the following chart. Samples 18A and 19A, made with the host material No. 2, exhibited the electrical characteristics listed in Table V. Material wt.% Glass wt.% Host wt.% Bi2O3.2TiO2 wt.%Mn(NO3)2 18A 1.199 88.813 9.783 0.205 19A 1.599 88.613 9.583 0.205
Electrical Properties Material 18A 19A Firing temp°C 1080 1100 1080 1100 diel.thk. mils 0.71 0.74 0.88 0.83 diel. constant. 3219 3311 2979 3218 % DF 1.82 1.89 1.66 1.70 -55°%Δ -7 8 -8 -9 85°%Δ -8 -10 -8 -10 125°%Δ -7 -12 -7 -11 25°R.C >10K >10K >10K >10K 125°R.C 2871 280 4825 495

From the results as listed in Table V, it can be seen that by increasing the level of glass present in the composition, there is a corresponding increase in the R.C product at 125°C. This measurement is of great value to capacitor manufacturers, with higher values generally indicating a more robust material less likely to suffer sudden catastrophic breakdown.

Finally, the following chart lists sample compositions 20A-24A which are made with host No. 2, and have respectively different levels of the 50% aqueous solution of Mn(NO3)2 referred to in the first-noted sample formulation. Also, Table VI reflects the electrical characteristics of these samples 20A-24A. Material wt.% Mn(NO3)2 wt.% Host wt.% Bi2O3.2TiO2 wt.% glass pdr 20A 0.00 89.196 10.004 0.800 21A 0.05 89.151 9.998 0.800 22A 0.100 89.107 9.993 0.800 23A 0.205 89.013 9.983 0.799 24A 0.305 88.924 9.973 0.798
Electrical Properties Material 20A 21A 22A 23A 24A Firing temp°C 1080 1100 1080 1100 1080 1100 1080 1100 1080 1100 diel.thk. mils 0.80 0.82 0.87 0.84 0.77 0.81 0.85 0.86 0.84 0.84 diel.const. 2949 3140 3259 3100 3096 3030 2890 2863 3177 3096 % DF 1.59 1.50 1.63 1.86 1.78 2.20 1.58 1.54 1.44 1.38 -55°%Δ -9 -8 -8 -10 -10 -17 -12 -10 -8 -6 85°%Δ -10 -11 -10 -11 -9 -10 -8 -10 -9 -12 125°%Δ -12 -16 -11 -16 -11 -15 -11 -14 -13 -16 25°R.C >10K >10K >10K >10K >10K >10K >10K >10K >10K >10K 125°R.C 1 1 20 14 127 1261 581 622 1967 267

From the foregoing it will be apparent that the present invention provides novel ceramic dielectric compositions which are particularly suitable for use in the production of multi-layer capacitors of the low fire variety, which are adapted to be fired at or below approximately 1100°C. Because the capacitors can be fired at such low temperatures, it is possible, as noted above, to utilize therefor substantially more inexpensive electrode materials, as compared for example with high fired varieties. The ability to fire the capacitors at relatively low temperatures results from the use of a special barium titanate host material mixed with a ceramic sintering aid produced from bismuth and titanium oxides or precursors thereof, a small amount of glass powder and a trace of manganese dioxide, or a precursor thereof. The host material, which comprises a mixture of barium titanate and neodymium oxide, when used in a formulation including the above-noted sintering aid, glass powder and manganese dioxide, results in suitable X7R formulations when the weight percent of the barium titanate in the host material is in the range if 97.89 to 98.19, and the neodymium oxide weight percent is in the range of 1.81 to 2.11. A typical formulation for producing such dielectric ceramic compositions comprises, in weight percent, the host material in an amount of 91.013, the sintering aid in the form of Bi2O3.2TiO2 in an amount of 7.983, glass powder in the amount of 0.799 and the manganese dioxide in the amount of 0.205. (In practice the glass powder itself may comprise 86.0 wt.% of PbO, 9.0 wt% B2O3, 1.58 wt.% SiO2, 0.13 wt.% TiO2 and 3.29 wt.% Al2O3.)

From the foregoing, it will be noted also that the barium titanate, which is used in the host material, should be precalcined at temperatures at least equal to or greater than 1000°C., or otherwise the formulated sample dielectric compositions will not meet the X7R specifications. Notably also, while the sample dielectric compositions produced by the above-noted formulation can be produced with dielectric constants in excess of 2700, nevertheless dielectric compositions having dielectric constants in excess of 3000, can be achieved by effecting only slight adjustments in the calcining temperatures for the host material, or by making slight variations in the amount of the Bi2O3.2TiO2 employed in the formulation with the host material. Similar such changes in the dielectric constant can be achieved by slight changes in the amount of the glass powder used in the formulation, and to a lesser extent by changes in the amount of manganese dioxide employed in the formulation.

Finally, it will be apparent from the foregoing that extremely reliable and less expensive, low fire multi-layer capacitors can be produced by utilizing the novel X7R dielectric compositions disclosed herein.

This invention has been illustrated and described in detail in connection with only certain embodiments thereof. Those skilled in the art will be able to make further changes.


Anspruch[en]
  1. A low fire ceramic dielectric composition in powder form capable of producing a multi-layer capacitor having a dielectric constant which does not vary more than 15% from its value at 25°C over a temperature range of -55°C. to 125°C., which composition comprises a barium titanate and neodymium oxide mixture forming a host material present in the range of 88.6 to 91 wt.% of the composition, the barium titanate constituting from 97.89 to 98.19 wt.% of the host material, and the neodymium oxide constituting from 1.81 to 2.11 wt.% of the host material; a Bi2O3.2TiO2 sintering aid present in the range of 7.98 to 10.0 wt.% of the composition; a glass powder present in the range of 0.8 to 1.6 wt.% of the composition; and optionally a trace amount of manganese dioxide.
  2. A composition according to claim 1, which also contains manganese dioxide in an amount up to 0.1% by weight of the composition.
  3. A composition according to claim 1, which also contains manganese dioxide derived from including up to 0.305 wt.% of a 50% aqueous solution of Mn(NO3)2 in the preparation of the composition.
  4. A composition according to claim 1, 2 or 3, wherein the barium titanate is calcined at a temperature in the range 1100°C to 1225°C before being mixed with the neodymium oxide.
  5. A composition according to claim 1, 2, 3 or 4, wherein the host material is calcined at a temperature in the range 1200°C. to 1365°C.
  6. A composition according to any preceding claim, wherein said Bi2O.2TiO2 is present in the range 9.983 to 7.983 wt.% of the composition, and the host material is present in the range 89.013 to 91.013 wt.%.
  7. A low fire multi-layer ceramic capacitor made from a composition as claimed in any of claims 1 to 6.
  8. A capacitor according to claim 7, which has a dielectric constant equal to or greater than 3000.
  9. A capacitor according to claim 8, which has been fired at a temperature of up to 1100°C.






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