DE19945014C1 - Reduction-stable X7R ceramic composition used for multi-layered capacitors with nickel inner electrodes is based on a core component made of barium titanate and different shell components - Google Patents

Abstract

Reduction-stable X7R ceramic composition is based on a core component made of barium titanate (BaTiO3) and different shell components. Reduction-stable X7R ceramic composition based on a core component made of BaTiO3 and different shell components has the following composition: ((BaTiO3)0.98(BaCo3)x(SrCo3)0.005(SEO1.5)y(MgO, NiO)0.005(MnO)0.003)0.98 (BaZrO3, SrZrO3, NdTiO3.5)0.02 + 0.5 wt.% ((MgO)0.506(B2O3)0.233(SiO2)0.261) SE = ytterbium, europium, yttrium, holmium, samarium, dysprosium or neodymium; x = 0-0.04; and y = 0.005-0.02.

Description

The invention relates to reduction-stable X7R ceramic materials high dielectric constant according to the generic term of Pa claim 1.

Such X7R ceramic materials are preferred at Viel layer capacitors with nickel internal electrodes are used and are for applications in electronics and electrical engineering suitable. With the use of nickel instead of the expensive one Palladium as a material for the internal electrodes is a knockout most advantage connected.

X7R capacitor ceramics are generally based on ferroelectric barium titanate. In order to reduce the temperature dependence of the dielectric constant, which is very pronounced in the area of the ferroelectric Curie temperature, the BaTiO ₃ powder particles as the core component “core” are modified by the addition of certain additives in such a way that a shell, “shell”, is at least partially around each grain. , arises and an inhomogeneous ceramic structure is created. X7R ceramics are materials that have a temperature coefficient of capacity of ± 15% in the temperature range from -55 ° C to + 125 ° C.

Technical solutions have already become known which allow the joint sintering of an X7R ceramic mass with Ni inner electrodes under reducing conditions at 1300 to 1350 ° C. In order to prevent the oxidation of the nickel in the heating phase and during the sintering, an oxygen partial pressure down to 10 ^-8 Pa, but at least 10 ^-2 Pa must be used. Under such conditions, BaTiO _{3 is} already partially reduced, which inevitably leads to a reduction in the insulation resistance and thus to an inadmissible increase in the dielectric losses. Therefore, the X7R capacitor ceramics suitable for Ni internal electrodes to increase the stability with respect to reduction, for example, an excess of the composition BaTiO ₃ in the "core", an excess of BaO in the "shell" area, which contains the other additives as Acceptors act, bind and bind with the BaTiO ₃ base, so that the formation of movable charge carriers, which reduce the resistance to insulation, is prevented even under the specified reducing conditions.

Sakabe [Ceramic Bulletin, 66 (1987) 1338] has described that nickel can advantageously be used instead of palladium in multilayer capacitors if the stability of the ceramic against reduction in an atmosphere p _O2 <10 ^-5 Pa by Incorporation of Ca ²⁺ or Mg ²⁺ acceptors on Ti ⁴⁺ sites in the crystal structure of BaTiO _{3 is} brought about by adding an excess of BaCO ₃ with the shell component to the ceramic mass, which in addition to the CaCO ₃ and / or MgO and other additives. According to the publication DE 42 20 681 C2, the shell component contains not only MgO but also MnO and, above all, an oxide of the rare earth metals SE ₂ O ₃ (SE = Dy, Ho, Er or Tb) and optionally also up to 2.5% by weight .-% of a glass frit, e.g. B. consists of a BaO-SrO-Li ₂ O-SiO ₂ glass.

In the publication EP 0 605 904 A2, a ceramic composition based on BaTiO _{3 is} described, which additionally contains cobalt. A glass system with the participation of lithium is added to this ceramic mass.

The publication US Pat. No. 5,397,753 A1 also proposes the addition of up to 3 mol% of BaZrO ₃ to the shell component in order to be able to produce stable X7R multilayer capacitors with Ni internal electrodes. Information on the essential accelerated life test HALT (Highly Accelerated Life Test) is provided by Y. Sakabe, Y. Hamayi and T. Nishiyama [Ferroelectrics, 133 (1992) 133] for such multilayer capacitors.

The procedure based on the publication EP 0 504 756 A1 is also based on BaTiO ₃ in the "core" and a complex composition in the "shell" area. Here, in addition to Ba CO _3, 1 to 3 mol% of MgO, ZnO, TiO ₂ and SE ₂ O ₃ (SE = Ho, Er, Yb, Dy or Y) are mixed with the BaTiO ₃ powder and the enveloping one by calcination at 1000 ° C "Shell" area trained in a first step. Only after the admixture of CaZrO ₃ or BaZrO ₃ and the frit of a silicate or borosilicate glass is the "shell" structure finally formed in the process of sintering the multilayer capacitors. In addition to its acceptor function, MgO has an inhibiting effect on the diffusion of the SE ^{3 +} cations in the lower "Shell" area directly adjacent to BaTiO ₃ , which promote the uncontrolled grain growth [H. Kishi, Y. Okino, M. Honda, Y. Iguchi, M. Imaeda, Y. Takahashi, H. Ohsato, T. Okuda: Jpn. J. Appl. Phys. 36 (1997) 5954]. In the HALT test (0.1 µF capacitor 0805 with 12 µm dielectric thickness at 350 V and 165 ° C), a lifespan of 1500 minutes was found in 50% of the parts of a typical batch [H. Kishi, N. Yamaoka, Science of Ceramic Interfaces II, J. Novotny (Editor) 1994 Elsevier, pages 613 to 616].

A process-technically simplified procedure is given in the documents US 5 403 797 A1 and 5 335 139 A1 and EP 0 615 262 A3. BaTiO ₃ powder of a certain dispersity is mixed with variable proportions of the "Shell" components [(MgCO ₃ ) ₄ Mg (OH) ₂ (H ₂ O) ₅ ], MnCO ₃ , V ₂ O ₅ and Y ₂ O ₃ 5% by weight of a mixture of BaCO ₃ , CaCO ₃ and SiO ₂ or a (Ba _1-x Ca _x ) SiO ₃ glass frit BCG (with, for example, x = 0.5) were added and immediately, that is, without the intermediate step of a calcination with subsequent grinding, processed into a slip from which foils for multilayer capacitors are drawn. The HALT reliability test carried out on such multilayer capacitors with Ni-In internal electrodes (at 180 ° C. under a voltage of 10 V / μm) gave R _IS <2 for the time until the insulation resistance fell below. 10 ⁵ Ω values of up to 2000 minutes.

From the document EP 0 814 486 A1 a mass concept is known which avoids the BaO excess. In addition to about 1 mol% of SE ₂ O ₃ (SE = Tb, Dy, Ho, Er or Yb), up to 4 mol% of a mixture of manganese oxide and nickel oxide and up to 5 mol% are used to build up the enveloping “shell” structure MgO and an addition of several mole percent SiO ₂ for use. The assessment of the reliability in the HALT test at 150 ° C and 100 V leads to comparatively high values under these mild conditions when determining the length of time until the case R _IS <10 ⁶ Ω has occurred. A possibility of comparison with the reliability data given above, which were obtained under tougher conditions, is practically not possible, since the mechanisms of resistance degradation can be temperature and voltage dependent.

A series is disclosed in EP 0 821 377 A2 whose silicate and borosilicate glasses are part of the "Shell" areas listed and related in this regard the meaning of phases containing manganese oxide and / or of Si licate phases in the vicinity of the Ni internal electrodes, since this of increasing aging stability and reliable multi-layer capacitors with Ni internal electrodes are such.

Based on this, the older ones are not pre-published ten publication DE 199 18 091 A1 reduction-stable X7R condensers masses have been described in the sinter densification in multilayer capacitors with Ni internal electrodes result in inhomogeneous structure, which a high reliability in HALT test.

The invention has for its object to provide a suitable X7R To specify capacitor ceramic mass, which with multilayer con capacitors with Ni internal electrodes to a comparatively high dielectric constant with high reliability liquid leads.  

This object is achieved with a reduction stabilizer len X7R ceramic mass solved that the characteristics of the patent claim 1 owns.

Advantageous further developments are in the dependent claims guided.

The advantage of the invention is that the phase build-up typical for X7R capacitor ceramics in the shell area with the additives BaCO ₃ , SrCO ₃ , an oxide of the rare earths SE ₂ O ₃ , MnO and basic magnesium carbonate or NiO with the addition of further additives such as BaZrO ₃ , SrZrO ₃ or Nd ₂ Ti ₂ O ₇ and a borosilicate glass frit can also be realized. As oxide component of the rare earth sector, Yb ₂ O _3, Y ₂ O _3, Dy ₂ O ₃ or Ho ₂ O ₃ and prove as the technology in the prior Tech have hardly contemplated Eu ₂ O ₃ as appropriate.

The increased stability against reduction in the sintering together with Ni internal electrodes is brought about by the BaO excess and the at least partial effect of the other additives as acceptors. The sinter densification is achieved by adding BaTiO ₃ as the core component with part of the shell components, namely MgCO ₃ or NiO, SrCO ₃ , BaCO ₃ , MnCO ₃ and an oxide of rare earths in a calcination step for 1 h 980 ° C to form an inhomogeneous structure, then carry out a fine grinding and add the additives Nd ₂ Ti ₂ O ₇ , SrZrO ₃ or BaZrO _3, which are subjected to separate grinding, to the slip before the film is drawn together with the glass frit.

The thermal fixation of the shell components in a cal zination step with subsequent suitable grinding and Dispersion avoids difficulties arising from the in the publications US 5 403 797 A1, US 5 335 139 A1 and Procedure described in EP 0 615 262 A2 reveals where on the calcination is dispensed with.

The invention is described below with reference to exemplary embodiments play explained in more detail.

Embodiments

As the core component C, barium titanate powders are suitable, referred to as C1, which are produced by reacting an equimolar mixture of BaCO ₃ and TiO ₂ at 1140 ° C. and a holding time of 4 hours. With a suitable morphology of the BaCO ₃ and TiO ₂ raw materials, a BaTiO ₃ powder with an average grain size of 1.5 µm is obtained. In the same way, BaTiO ₃ powders, referred to as C2, are suitable which, as a result of thermal decomposition of BaTiO (C ₂ O ₄ ) ₂ . 4H ₂ O arise. These typically have an average grain size of approx. 0.8 µm.

Combined additives which, in addition to BaCO ₃ , SrCO ₃ , an oxide of the rare earths SE ₂ O ₃ and optionally manganese oxide, also contain NiO or MgO and with the BaTiO ₃ in the core area by calcination at 980 ° C. are advantageous as shell component S. (Holding time 1 h) chemically. The reaction product is then subjected to a grinding process, an average grain size of 0.7 μm being aimed for.

To complete the shell structure S in the subsequent sintering process, the ground product is further additives BaZrO ₃ or optionally SrZrO ₃ or Nd ₂ Ti ₂ O ₇ and a glass frit consisting of a glass of the MgO-SiO ₂ -B ₂ O _{3 system} in finely dispersed form (average grain size approx. 1 µm). After intensive mixing, the ceramic mass is either processed into a granulate for the purpose of compression molding into disk-shaped samples, which allow the dielectric properties to be measured after sintering, or a slip is prepared with the addition of dispersant and binder, which is used to draw a film with subsequent build-up of multilayer capacitors with Ni internal electrodes.

In this way, slice samples and multilayer capacitors with nickel internal electrodes are assembled in a composition of the general formula:

[(BaTiO ₃ ) _0.98 (BaCO ₃ ) _x (SrCO ₃ ) _0.005 (SEO _1.5 ) _y (MgO, NiO) _0.005 (MnO) _0.003 ] _0.98 [BaZrO ₃ , SrZrO ₃ , NdTiO _3.5 ] _0.02 + 0.5% by weight [(MgO) _0.506 (B ₂ O ₃ ) _0.233 (SiO ₂ ) _0.261 ]

receive. The figures are fractions of moles. The glass frit was only added to 0.5 percent by mass.

Debinding and sintering of the slice samples was carried out at 1350 ± 50 ° C in air, to determine the stability of the ceramic against reduction on a trial basis even under nitrogen (p _O2 ≈ 10 ^-3 Pa). Ag paste was used for contacting, baking was carried out at 800 ° C. (1 h).

Table 1 gives information on variants of the production of the X7R ceramic mass. The sintering atmosphere was air at No. 2 N ₂ . The additive was 0.02 mol and Ba / Ti _is listed _overall .

The examples with the numbers 1, 2, 12, 13 and 15 according to Tabel le 1 are not examples according to the invention but only Comparative examples.  

Table 1

Table 2 shows the measured values of the dielectric properties of disk samples after sintering at 1350 ° C./3 h and contacting by baking Ag electrodes in air. The dielectric constant ε _eff and the loss factor tanδ were measured at 25 ° C and 1 kHz measuring frequency. The TKC curve is based on an ε at 25 ° C as a reference. Sample diameter approx. 13 mm, sample thickness approx. 1 mm

Table 2

Reproducibility and stability of the ceramic during sintering under greatly reduced partial pressure of oxygen are demonstrated by samples 1 (air) and 2 (N ₂ ). It can be seen that the property criteria for an X7R ceramic are met.

Through the additional installation of MnO in the shell assembly If 1 is set in offset 3, the loss factor tanδ si significant. The temperature dependence of the capacity in the loading range from -55 ° C to 125 ° C is advantageous compared to 1 reduced. The addition of MnO is an improvement the sintered density is reached. At the sintering temperature of 1350 ° C The resulting oxygen vacancies will cool down or tempering in an atmosphere with defined oxygen partial pressure replenished, which improves the dielectri properties of ceramics.  

If Yb ₂ O ₃ of ceramic sample 3 is replaced by Y ₂ O ₃ (4) or Ho ₂ O ₃ (5) while the composition is otherwise the same, dielectric properties also result which correspond to the criteria defined for an X7R capacitor ceramic. The use of BaTiO ₃ C2 (d ₅₀ ≈ 0.8 µm) in FIG. 6 shows that when this finely dispersed core component is used, the homogenization in the ceramic proceeds too far and thus leads to the TKC tolerance range being exceeded.

If otherwise the same composition is used instead of Yb ₂ O ₃ , Y ₂ O ₃ or Ho ₂ O _3, the rare earths Nd ₂ O ₃ (7), Sm ₂ O ₃ (8) or Eu ₂ O ₃ (9) , there are sometimes large deviations from the TKC criteria required for X7R capacitor masses. A gradual reduction in the content of Eu ₂ O ₃ at 10 and 11 again leads to a TKC course at offset 11, which fulfills the X7R criteria within the required tolerance of ± 15%. This proves the production of a redox-stable ceramic mass with Eu ₂ O ₃ , which fulfills the electrical parameters specified for X7R.

The offset 12 shows that the property values also the Er meet requirements if the to guarantee Re DOX stability required acceptor addition MgO by NiO is exchanged.

Samples 13, 14 and 15 show that the additive BaZrO ₃ used in the other samples can also be replaced by Nd ₂ Ti ₂ O ₇ or SrZrO ₃ , the TKC criteria only being met in the latter case.

The increase in the Ba / Ti ratio from 1.01 to 1.04 at 14 is associated with a strong reduction in the sinterability, which gives rise to a decrease in the R _IS value and an increase in the loss factor. At batch 14, 0.4 mol% of H ₃ BO ₃ and 0.6 mol% of SiO ₂ were therefore added in addition to the addition of 0.5 m% of glass frit, and sinter compaction was achieved in this way.

The suitability of the ceramic masses produced for the application in multilayer capacitors with Ni internal electrodes a selection of the types listed in Tables 1 and 2 sentences occupied.

The slip required for drawing the ceramic foils is produced from the powder mixture ground to a certain dispersity using water with the aid of a dispergator. A solids content of 65 m% is aimed for and the viscosity is adjusted to values of 20 to 30 Ma s with the addition of an acrylate binder. After printing the foils with a commercially available Ni-Pa ste of a metal content of 60 m%, stacking, laminating and cutting, the individual parts are subjected to the process of debinding by gradual heating up to 500 ° C. in a nitrogen atmosphere, whereby initially a residual carbon content of approx. 0.5% remains in the ceramic. The elimination of the residual carbon and the subsequent sintering of the debindered components is accomplished by heating to 1300 to 1360 ° C. in a nitrogen atmosphere with the addition of a CO ₂ / H ₂ process gas mixture which, depending on the composition, has the desired oxygen partial pressure between 10 ^-3 adjustable up to 10 ^-8 Pa. In order to heal defects due to the beginning of reductive degradation of the ceramic during sintering, tempering is carried out in the heating phase at an oxygen partial pressure of up to 1 Pa. The termination of the multilayer capacitors with Ni internal electrodes obtained in this way is carried out with Cu paste according to a standard method by briefly heating up to 935 ° C (baking), with a reduction of the ceramic due to organic contained in the termination paste Components must be avoided, what z. B. succeeds in that a hydrogen-water vapor mixture is supplied to the nitrogen atmosphere.

The temperature-time diagram 1 shown in Fig. 1 shows the process control for sintering capacitors of the mass composition sample No. 3 in various variants A to F.

In stage I water vapor is already added to the N ₂ stream and H ₂ is additionally added in II in order to completely eliminate the residual carbon at 800 ° C. and to avoid oxidation of the internal electrodes. With the exception of variant A, an H ₂ / CO ₂ process gas mixture corresponding to the indicated oxygen partial pressure is then metered in during ramp III. Sintering takes place at 1300 ° C or 1360 ° C in section IV at the listed oxygen partial pressure. In stage V, the p _O2 value is ultimately retained unchanged or set to a specific value. The set partial pressure is identical to that specified in ramp III.

The thermal post-treatment (stage VI) arrives in the given way of application. To the influence of an oxidative Aftertreatment to the dielectric property behavior parallel capacitors without thermi after-treatment (without stage VI).

Tables 3 and 4 list the electrical parameters of multilayer capacitors according to mass composition sample No. 3 and type 0805/50 V / 100 nF, as they were obtained after the various variants of the atmosphere control during sintering at 1360 ° C. The capacitors contain 50 printed layers. Due to the manually performed metallization, the capacitance values understandably show a certain spread. The sintered dielectric layer thickness is 15 µm. The effective dielectric constant ε _eff can be calculated from the capacitance using the formula C = (n - 1) ε ₀ ε _eff A / d. The capacitance values were measured at 1 kHz and to determine the temperature coefficient TKC in the temperature range from -55 ° C to 125 ° C. The insulation resistance was determined with an applied voltage of 50 V. In the property values shown, τ is given as the product of capacitance and insulation resistance.

Table 3

Table 4

The associated reliability measurement data are compiled in Table 5. The "Highly Accelerated Life Time" test HALT makes a statement about the reliability of the capacitors and is carried out by the components z. B. at 140 or 200 ° C with a voltage between 140 to 200 V, usually with four times the nominal voltage, and the duration is determined up to that at 70% (column a; R _{IS, 50%} <0.7R _{IS; beginning} ) or 50% (column b; R _{IS, 50%} <0.5R _{IS; beginning} ) of the components of a batch the insulation resistance decreases to 0.7 or 0.5 of the initial value or simply determines until at what time the insulation resistance of the components <1 MΩ (column c; R _{IS, 50%} <10 ⁶ Ω) remains.

Table 5

It can be seen that the X7R criteria for the after sintering regime A manufactured capacitors are almost met, and themselves the use of the oxidative tempering stage VI advantageous works.

The results show that the entrainment of the process gas from 800 ° C (end of decarburization) is advantageous for improving the reliability because in this way a partial nickel oxidation is avoided and the formation of oxygen vacancies as a result of the dissolution of NiO in BaTiO _{3 is} prevented. The experimental results show that at 1360 ° C / 2 h, the same heating and cooling rate and with decreasing oxygen partial pressure, an increase in the dielectric constant and the loss factor is achieved. At the same time, the dependence of the capacitance on the temperature, the TKC value, increases. Partial oxygen pressures below 10 ^-8 Pa (1360 ° C) should be avoided when manufacturing condensers of sample No. 3.

From the disk experiments in Tables 1 and 2, based on the results of approach # 3, further multilayer capacitors of the 0805 design provides, whose composition according to sample no. in Table 1 in Brackets are listed.

Their manufacturing conditions (process control P, sintering temperature temperature ST in ° C and holding time H in h) and electrical parameters ter are in Tables 6, 7, and the information on the ver Reliability after HALT measurements in Table 8.

The process control also corresponds to the temperature-time diagram shown in FIG. 1. On the basis of the positive findings when using the thermal aftertreatment in an atmosphere with increased oxygen partial pressure, stage VI was applied consistently.

The capacitors described here each contain 20 be printed layers

Table 6

Table 7

Table 8

Sample no. with mass composition acc. Ta belle 1, process control P, sintering temperature ST (° C) / holding time H (h), dielectric thickness d in µm, HALT conditions B (° C / V) nd HALT measurements a, b, c acc. Table 5.

It can be seen that the X7R criteria are achieved in conjunction with a favorable value for the reliability in the case of capacitors of the ground composition 6. In this experiment, after the conversion of the fine-grain BaTiO ₃ (C2) with BaCO ₃ , SrCO ₃ , MgCO ₃ , MnO and Ho ₂ O ₃ at 980 ° C (1 h), a stage of fine grinding (d ₅₀ = 0.7 µm) ) applied and the separately manufactured and also ground additives BaZrO ₃ and magnesium borosilicate glass were added during the preparation of the slurry.

Compared to the multilayer capacitors of mass composition 5, which were processed under the same conditions, the dielectric constant ε _{eff is} significantly increased in mass composition 6, and the τ value is far above the value of 1000 seconds required in the X7R characteristic. 5 and 6 are comparable in the reliability test.

By substituting Ho ₂ O ₃ (0.02 mol) in multilayer components 5 and 6 with Eu ₂ O ₃ (0.005 mol) in the shell area of components acc. Mass composition 11, the electrical parameters of the X7R criteria are also met. The values for the reliability of the capacitors are consistently much lower.

The substitution of BaZrO ₃ by SrZrO ₃ in the capacitors with mass composition 13 or 14 results in a particularly low temperature dependence of the capacitance in the range from -55 ° C to + 125 ° C. The ceramic materials would also meet the requirements of X7F capacitors. However, the reliability values in the HALT test are inadequate.

Claims (7)

1. Reduction-stable X7R ceramic mass of high dielectric constant, in particular for multilayer capacitors with nickel internal electrodes, built according to the core-shell concept based on BaTiO ₃ as core component and various shell components, characterized by a composition corresponding to the general For mel
[(BaTiO ₃ ) _0.98 (BaCO ₃ ) _x (SrCO ₃ ) _0.005 (SEO _1.5 ) _y (MgO, NiO) _0.005 (MnO) _0.003 ] _0.98 [BaZrO ₃ , SrZrO ₃ , NdTiO _3.5 ] _0.02 + 0.5% by weight [(MgO) _0.506 (B ₂ O ₃ ) _0.233 (SiO ₂ ) _0.261 ]
with SE = Yb, Eu, Y, Ho, Sm, Dy or Nd and 0 ≦ x ≦ 0.04 and 0.005 ≦ y ≦ 0.02. 2. Reduction-stable ceramic mass according to claim 1, characterized by the use of BaTiO ₃ powder of an increased average dispersity as the core component. 3. Reduction-stable ceramic mass according to claim 1 or 2, characterized in that the X7R ceramic has a phase-heterogeneous structure by building around the barium titanate in the core a shell area, the composition of which by binding SrCO ₃ with MgO and an oxide component the rare earths SE ₂ O ₃ with SE = Yb, Eu, Y, Ho, Sm, Dy or Nd come about with the participation of manganese oxide in BaTiO ₃ , and which is also fritted by BaZrO ₃ and by adding a magnesium borosilicate glass frit is modified. 4. reduction-stable ceramic mass one of claims 1 to 3, characterized,  that the phase heterogeneous structure of the X7R ceramic mass in Shell area comes about that MgO by NiO er sets is. 5. Reduction-stable ceramic composition according to claim 3 or 4, characterized in that the phase-heterogeneous structure of the X7R ceramic composition in the shell area is achieved in that BaZrO _{3 is} replaced by SrZrO ₃ . 6. Reduction-stable ceramic mass one of claims 1 to 5, characterized by the addition of 0.5 wt .-% of a glass powder of the system MgO-B ₂ O ₃ -SiO ₂ with a grain size between 0.8 and 1.2 µm as a sintering aid . 7. Use of a reduction-stable ceramic mass according to egg nem of claims 1 to 6 for X7R capacitors with Ni-In internal electrodes.