Auxiliary agent for sintering ceramic material

An auxiliary agent for sintering ceramics comprises a basic composition consisting essentially of (a) 5 to 40 mol % of at least two oxides selected from the group consisting of ZnO, SnO and MnO, the sum of the contents of ZnO and MnO being not less than 5 mol %, (b) 5 to 40 mol % of at least one metal oxide expressed by the general formula: RO where R is at least one element selected from the group consisting of Ba, Sr, Ca and Mg, and (c) 30 to 70 mol % of (Ti, Si)O.sub.2, the content of SiO.sub.2 being not less than 15 mol %. The auxiliary agent may contain at least one additive selected from the group consisting of Al.sub.2 O.sub.3, B.sub.2 O.sub.3, CuO, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, Ga.sub.2 O.sub.3, GeO.sub.2, Y.sub.2 O.sub.3, ZrO.sub.2, WO.sub.3, MoO.sub.2, Fe.sub.2 O.sub.3, CoO, NiO, PbO and Bi.sub.2 O.sub.3. The total content of the additive is not more than 20 parts by weight per 100 parts by weight of the basic composition, and the content of at least one metal oxide selected from the group consisting of Y.sub.2 O.sub.3, ZrO.sub.2, WO.sub.3, MoO.sub.2, Fe.sub.2 O.sub.3, CoO, NiO, PbO and Bi.sub.2 O.sub.3 should be not more than 10 parts by weight per 100 parts by weight of the basic composition. The auxiliary agent may further contain 2 to 45 mol % of Li.sub.2 O. SnO and MnO may be omitted from the basic composition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an auxiliary agent for sintering ceramic 
materials and, more particularly, an auxiliary agent for sintering 
dielectric ceramic materials. 
2. Description of the Prior Art 
In general, ceramic dielectrics mainly comprising barium titanate 
(BaTiO.sub.3), or strontium titanate (SrTiO.sub.3), or titanium oxide 
(TiO.sub.2) or the like have widely been used for production of monolithic 
ceramic capacitors. Such monolithic ceramic capacitors are generally built 
up of an arrangement of alternating two layers of a dielectric ceramic 
layer and an internal electrode layer and are produced by preparing 
ceramic green sheets, forming a layer of metal paste for internal 
electrodes on each ceramic green sheet, stacking them and firing the 
resultant stacked body at a temperature of 1300.degree. C. and above. It 
is therefore required to use a high-melting noble metal such as 
platinum-palladium as a material for internal electrodes to prevent from 
oxidation. 
However, the noble metals are expensive and occupy a greater part, 
generally about 30 to 50%, of the production cost of monolithic ceramic 
capacitors. Thus, the use of the noble metal sets a limit to reduce the 
production cost of the monolithic ceramic capacitors. In addition, a 
sintering furnace is considerably damaged at high sintering temperatures 
of not less than 1300.degree. C., resulting in increase of maintenance 
cost of the furnace with the operating time being elapsed. Further, the 
sintering at high temperatures results in considerable increase of the 
energy cost required for production of monolithic ceramic capacitors. 
To solve such problems, it has been proposed to use a lead-containing 
dielectric ceramics with a low-sintering temperature or nonreducible 
dielectric ceramics, as a material for capacitors. These ceramic materials 
make it possible to use cheap metals such as nickel as a material for 
internal electrodes of the monolithic capacitors. Also, attempt has been 
proposed to produce monolithic ceramic capacitors by a process comprising 
the steps of preparing multilayer sintered ceramic bodies with plural 
spaces between adjacent ceramic layers, pouring a molten metal such as 
lead or a lead-tin alloy into each space to form internal electrodes, and 
cooling the same to solidify the molten metal. However, such new 
dielectric ceramic materials have some disadvantages awaiting a solution. 
On the other hand, the dielectric ceramic compositions of the prior art 
have excellent dielectric properties required for production of ceramic 
capacitors including monolithic ceramic capacitors. Thus, if the 
dielectric ceramic compositions of the prior art can be sintered at low 
sintering temperature, there is no need to use the above new dielectric 
ceramic materials. 
SUMMARY OF THE INVENTION 
It is therefore a main object of the present invention to provide a 
auxiliary agent for sintering dielectric ceramics that makes it possible 
to lower the sintering temperature of dielectric ceramic compositions 
without causing deterioration of electrical properties thereof. 
According to the present invention, the above and other objects are solved 
by providing a auxiliary agent for sintering ceramics, that comprises a 
basic composition composed of 
(a) 5 to 40 mol % of at least two oxides selected from the group consisting 
of ZnO, SnO and MnO, the sum of the contents of ZnO and MnO being not less 
than 5 mol%, 
(b) 5 to 40 mol % of a metal oxide expressed by the general formula: RO 
where R is at least one metal selected from the group consisting of oxides 
of Ba, Sr, Ca and Mg, and 
(c) 30 to 70 mol % of (Ti, Si)O.sub.2, the content of SiO.sub.2 being not 
less than 15 mol %. 
In the above basic composition, SnO and MnO of the group (a) may be omitted 
therefrom. Further, the above basic composition is incorporated with 2 to 
45 mol % of LiO.sub.2 as the occasion demands. 
In a preferred embodiment, the above basic composition is incorporated with 
at least one additive selected from the group consisting of Al.sub.2 
O.sub.3, B.sub.2 O.sub.3, CuO, Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, 
Ga.sub.2 O.sub.3, GeO.sub.2, Y.sub.2 O.sub.3, ZrO.sub.2, WO.sub.3, 
MoO.sub.2, Fe.sub.2 O.sub.3, CoO, NiO, PbO and Bi.sub.2 O.sub.3. The 
preferred content of the additive is not more than 20 parts by weight per 
100 parts by weight of the basic composition. However, the content of at 
least one metal oxide selected from the group consisting of Y.sub.2 
O.sub.3, ZrO.sub.2, WO.sub.3, MoO.sub.2, Fe.sub.2 O.sub.3, CoO, NiO, PbO 
and Bi.sub.2 O.sub.3 should be not more than 10 parts by weight per 100 
parts by weight of the basic composition.