FR2535313A1 - Process for the preparation of ceramics for multilayer capacitors. - Google Patents

Abstract

Process for the manufacture of sintered ceramic material based on oxides: (1-x)Pb(Mg1/3Nb2/3)O3 + xPb(Mg1/3Zn2/3)O3, with 0</= x < approximately 0.15. The sintering operation comprises: - a temperature rise at the rate of approximately 250 DEG C/h up to approximately 500 DEG C; - a plateau at approximately 500 DEG C until virtually all the binder has been removed; - recommencement of the temperature rise at the rate of approximately 250 DEG C/h up to a temperature of 820-840 DEG C; - lowering of the temperature to approximately 200 DEG C; - rapid temperature rise at the rate of approximately 20 to 30 DEG C/minute up to approximately 1000 DEG C and maintaining substantially this temperature for approximately 6 hours to bake the material. Application to multilayer capacitors.

Description

 The present invention relates to the manufacture of ceramics for multilayer capacitors.

The ceramic capacitor has become very important economically because its development is closely related to that of microelectronics.
Since the breakdown voltages of the ceramic capacitors are very high, the thicknesses of the material used can be very small (20 μm) without this modifying the dielectric characteristics. Thus, from 1960 appeared multilayer capacitors, made by techniques called "Cerfeuil" (Thomson LCC), equivalent in the dielectric plane to a large number of capacitors in parallel, which allows to obtain important values of capacity (several hundred pF) while keeping small dimensions.

The technology currently used is based on an optimization of the ideas of the 1960s 9 a dielectric ribbon of a few tens of microns thick is cut into sheets, the electrodes are screen printed on one side and the sheets are stacked to give a "millefeuille" in which capacitors of a few mm2 are cut
2 to a few cm of surface. Since the internal electrodes undergo the same sintering cycle as the ceramic (generally based on Gap03), they must be generally in palladium or platinum, which do not oxidize and do not diffuse in the ceramic at the temperature of 1.3500C essential to obtain a dense and coherent ceramic.

 The soaring price of precious metals has caused an upheaval in the field of multilayers, and the price of reinforcement can reach 90 Ó of the price of the capacitor; Ceramists must therefore propose less expensive solutions for the "general public" market.

One possibility was to use new dielectrics. Research has focused on low-temperature sintering compositions (900-10000C), which would allow the use of relatively inexpensive metals for electrodes (Ag, Cu). Solid solutions between perowskite or pyrochlore compounds based on lead oxide: Pb (Fe2 / 3W1 / 3) O3, Pb (Mg1 / 3Nb2 / 3) O3, Pb (Fe1 / 2Nb1 / 2)
O3, Pb (Zn1 / 3Nb2 / 3) O3 and having a high dielectric constant (greater than 10,000) are the most promising
The study of such materials, already begun previously, was taken again in 1980 in the laboratory of the plaintiff. The mechanism of formation of the phases perowskites of type Pb (AB) 03, and the influence of the distribution of the phases and the microstructure on the electrical characteristics of ceramics (dielectric constant, losses and resistivity) have been studied (see Study of dielectric ceramics based on lead oxide: Pb (Mg13Nb2 / 3) O3 and Pb (Fe1 / 2). Nb1 / 2) 03: by M. Leeune and JP Boilot, International Symposium on the New Orientations of Passive Components, Paris, March 29, 1982).

In this regard, see also: Polyphase dielectric ceramics for type II capacitors based on perowski and pyrochlore lead, by G. Deegardin, H. Bali and
B. Rameau, (Symposium cited).

The technique for producing ceramics is described in detail in the aforementioned documents and comprises the following steps
- dosage
- mixing-grinding
- drying
- chamotte
- grinding
- drying
- pressing
- sintering.

It has now been found that the properties and characteristics of the ceramics obtained can be substantially improved, and made compatible with internal Ag-Pd alloy electrodes (in a ratio of about 7030, for example), which constitutes significant technical and economic benefits,
The subject of the invention is essentially a process for the production of sintered ceramic material based on oxides (1 ×) Pb (Mg 13 Nb 23) 03 + x Pb (Mg / Zn 2/3) O 3, with x <0.15 approximately, characterized in that the sintering operation comprises: - - a rise in temperature at the speed of 2500C /
hour approximately up to 5000C,
- a plateau at 500 C until pra elimination
tically total of the binder,
- recovery of temperature rise to speed
from 2500C / hour to a temperature of 820-840C,
- lowering the temperature up to 2000C
ron,
- rapid rise.in temperature, at the speed of 20
300C / minute, up to about 10000C and hand
stay at this temperature for approx.
6 hours for baking the material.

 In practice, thermogravimetric analysis or any other similar technique is used to determine the duration that must be the first bearing at 5000C intended for the binder. This duration can be about half an hour, without this being limiting. The binder that must be removed is polyvinyl alcohol, and more specifically a polyvinyl alcohol solution in water.

 The lowering of the temperature from the temperature of about 820-8400C to a temperature of about 200 ° C or less can be done under cooling, but in practice it is achieved only through the thermal inertia of the oven, after cutting of it.

 Indeed, it has been found that by operating in this way a ceramic material having improved dielectric characteristics is obtained.

 Moreover, this process makes it possible to produce the desired ceramic materials without any addition, which constitutes an additional technological and economic advantage.

 It has also been found that, against all odds, the dielectric characteristics of the ceramics, such as the above, are further improved by carrying out the sintering, either in accordance with the above process, or even according to the conventional method, in a neutral gas atmosphere, in particular under a sweep. nitrogen.

 The invention therefore also relates to a process for the manufacture of sintered ceramic material as aforesaid, in which the sintering operation is carried out under a neutral gas atmosphere, advantageously under a nitrogen sweep.

 The invention also relates to a process for the manufacture of multilayer capacitors, in particular sandwich disk capacitors, composed of a plurality of units of the above-mentioned ceramic dielectric compositions (of a thickness of about 0.5 mm, for example ) separated from each other by electrodes made of metal or non-noble alloy, especially 30Pd-70Ag alloy, the assembly being sintered as indicated above, the dielectric characteristics of the ceramics used in the composition of the multilayer capacitors thus produced do not undergo noticeable alteration.

 As a variant, the sintering of all the dielectric material and the electrodes may be conducted in a neutral gas atmosphere, for example under a nitrogen sweep; the internal electrodes can then advantageously be made of nickel or copper-nickel alloy.

The materials thus produced have dielectric characteristics which make it possible to use them in capacitors of Z5U type [K20 C: 5000-10000, Tg # 20 C, 104 <250, @ 20 C # 10 "#, cm, #C (+ 10, + 85 C) 1 + 20,
C20 C
-56%]
These characteristics are obtained for the systems.

of the aforementioned oxides, without having to add, which constitutes an additional technological and economic advantage.

 The invention is described more concretely in the examples below, which does not limit it in any way.

EXAMPLE 1 (1-x) bp (Mg1 / 3Nb2 / 3) O3 + X Pb (Zn1 / 3Nb2 / 3) with x = 0.1.

1) Dosage.

Taking into account the stoichiometry of this composition and the characteristics of the raw materials, a balance of precision of the appropriate quantities of the following products was weighed:
- lead oxide (II) for analysis, Merck, 99% minimum,
optically pure niobium oxide (V), Merck, at 99.9%
minimum,
- magnesium hydroxycarbonate for analysis, Merck, at
58% of MgO.

 - zinc oxide for analysis, Merck, at least 99%.

 These components were dry blended in the appropriate proportions into a PVC container and homogenized for 1 hour.

 2) Grinding.

The resulting mixture was milled using a commercial chopper under the following conditions:
steel tank and blade
grinding balls in Zr 02 (diameter 2mm)
t = 45 min. V = 300 rpm
grinding liquid: acetone.

 This operation made it possible to obtain a powder having an average particle size of 1.5 μm and a specific surface area of 2.5 m 2 / g.

3) Drying
After the wet milling, intensive drying was carried out under the epiradiator so as to guarantee the homogeneity of the mixture.

4) Camouflage
On the powder obtained after drying, a chamotte or pre-sintering was carried out at 8O00C for 2 h 30 in alumina nacelles.

 5) Grinding.

 The powder was agglomerated after the champing operation, grinding was done in a PUC container, with stirring in a three-dimensional mixer in the presence of acetone, using ZrO 2 beads.

6) Drying
We operated as indicated under 3).

 7) Addition of a dressage binder.

 Before pressing, the powder was coated by adding Rhodoviol (trade name of a soluble polyvinyl alcohol), at 3% by weight relative to the total weight of powder of a solution at 10X by weight of this product. produced in water. This addition of binder was carried out on the powder previously put in the form of aqueous gypsum, itself kept under stirring, in order to ensure a better distribution of the binder.

8) Preasage
The thus treated slurry was dried and then sieved through a 350 μm sieve.

The pressing phase proceeded as follows
- pressure rise: 1 min.

- P max - 29.4107 Pa
- application time of Pmax = l min.

 - diameter of the pellets: 20 mm.

9) Sintering
The pellets were sintered in an alumina nacelle in a lanthanum chromite element furnace (temperature set at + 2 ° C.).

The cooking thermal CyClE was as follows:
- temperature rise up to 5000C, at the speed of
2500C / hour,
- bearing about 1/2 hour at 5000C,
- recovery of the rise in temperature, at the speed of
250 C / hour, up to 8300C,
- Lowering the temperature (by cutting the oven and simply cooling it under the effect of thermal inertia) to less than 2000C,
- rapid rise in temperature (20-300C / min) up to 1000 C and maintained at this temperature for 6 hours.

The sintered product obtained had the following dielectric characteristics
dielectric constant K2OoC = 10892,
K max = 12140 at Tc = 4 C, tg6 = 24.10-4 at 20 ° C.,
# C / C (+ 10C, + 85C) = + 8% - 57%, and # 20 C = 1011 # cm.

EXAMPLE 2
By following the procedure indicated in Example 1, but omitting zinc oxide (x = O), a Pb-based ceramic material (Mg1 / 3Nb2 / 3) O3 corresponding to the dielectric characteristics was likewise obtained. following dielectric constant K20 C = 9730,
Kmax = 12,200 for Tc = -100C, tg # = 14.10-4 and # 20 C = 8.1010 # cm, #C (+ 10C, + 85C = + 10%, - 57%.

C 20 C
EXAMPLE 3 (Comparative) = + 10%, 57%.

Operating as in Example 1, but performing a conventional sintering comprising - a rise in temperature at 250 C / hour up to 5000C, - a step of about 1/2 hour at 500 C, - a recovery of the temperature rise at speed
from 25O0C / hour approximately to 10000C, with
this temperature for 6 hours,
A ceramic material having the following dielectric characteristics was obtained
K 200 C = 7405.

(K max = 9392 for TC = 100C) tg # 20 C = 11.10-4.

[20 C = 6.1010 # cm]
#C (+ 10, + 85C) = 10%, - 55%.

 C 20 C.

Example 4

Operating as in Example 3, but carrying out the heat treatment cycle under nitrogen sweep, the following dielectric characteristics were obtained K20oc = 8826,
K max = 11029 for Tc = -8 C.

tg # 20 C = 15.10-4 and [20 C. = 9.10 10 cm]
#C (+ 10, + 85C) = + 10%, - 56%.

Claims (5)

 1. Process for the production of sintered ceramic material based on oxides (1-x) Pb (Mg1 / 3Nb2 / 3) O3 + x Pb (Mg1 / 3Zn2 / 3) O3, with # x <0.15, comprising the following conventional steps of dosing, mixing-grinding, drying chamottageg grinding, drying, pressing and sintering, characterized in that the sintering operation comprises  - a rise in temperature at the speed of 25O0C / hour  approximately up to 5000C approximately,  - a bearing at 5000C approximately until practical elimination  total amount of binder,  - recovery of the rise in temperature at the speed of  250tC / hour approximately to a temperature of 820-840 C,  - lowering the temperature to about 2000C,  - rapid rise in temperature, at a rate of about 20-300 ° C./minute, up to about 100 ° C.  sibly of this temperature for about 6 hours  for cuission of the material.  2. Method according to claim 1, characterized in that determines the duration that must have the first bearing at 500 C by thermogravimetric analysis.  3. Process for the manufacture of multilayer capacitors using the method according to one of claims 1 or 2, characterized in that it also employs internal electrodes of non-noble metal or alloy, especially 30Pd-70Ag alloy.  4. Process according to claim 3, characterized in that the sintering is carried out in a neutral gas atmosphere.  5, Process according to claim 4, characterized in that the sintering under nitrogen sweep is carried out and implemented internal electrodes nickel or copper-nickel alloy.