Ceramic Capacitor and Dielectric Compositions

This invention relates to ceramic capacitors and in particular to dielectric compositions for use in ceramic capacitors, for example ceramic disc capacitors.

According to one aspect of the present invention there is provided a capacitor dielectric composition comprising barium titanate (BaTiO.sub.3), niobium pentoxide (Nb.sub.2 O.sub.5) and gadolinium sesquioxide (Gd.sub.2 O.sub.3).

According to another aspect there is provided a method of manufacturing a ceramic capacitor including the steps of forming a disc-shaped or tubular-shaped ceramic body from a dielectric composition comprising barium titanate (BaTiO.sub.3), niobium pentoxide (Nb.sub.2 O.sub.5) and gadolinium sesquioxide (Gd.sub.2 O.sub.3), firing the ceramic body, providing separate opposed surfaces of the ceramic body with a respective metal coating, connecting a respective connecting wire to each metal coating and encapsulating the ceramic body in an insulating material.

We have found that small additions of niobium pentoxide (Nb.sub.2 O.sub.5) and gadolinium sesquioxide (Gd.sub.2 O.sub.3) to barium titanate (BaTiO.sub.3) produce excellent flat (i.e. low variations in the temperature coefficient of capacitance (TCC) from -25.degree. C. to 85.degree. C.), mid-K dielectric compositions. Mid-K dielectrics are those with dielectric constants of the order of 4000 and generally in the range 3000 to 5000. Barium titanate alone has a dielectric constant that is dependent on purity and crystallite size as well as its crystal structure which transforms at 0.degree. C. and 125.degree. C. These transformations are accompanied by sharp rises in dielectric constant such that the TCC from -25.degree. C. to 85.degree. C. can vary by .+-.100%. For example, a composition comprising 97.9% by weight (BaTiO.sub.3) 1.1% by weight Nb.sub.2 O.sub.5 and 1% by weight Gd.sub.2 O.sub.3 (the composition constituents being milled together in a conventional manner) had, subsequent to firing at 1370.degree. C., a dielectric constant value (K) of 4000 and TCC from -35.degree. C. to 85.degree. C. in the range .+-.20%. Such a value of K is higher than usually obtained for this TCC. Hitherto such TCC values were obtained for K values of the order of 2500.

In the following table the results for a number of dielectric compositions, having different proportions (by weight) of the constituents BaTiO.sub.3, Gd.sub.2 O.sub.3 and Nb.sub.2 O.sub.5, subsequent to firing at 1370.degree. C. for one hour are quoted.

Disc ceramic capacitors generally employ dielectric materials with K of the order of 2500 for the 2D4 class. The above table shows that there is a range of compositions based on barium titanate and having small additions (up to 1.5%) of gadolium sesquioxide and niobium pentoxide which are particularly useful for disc ceramic capacitors, although they may be suitable for other capacitor types, for example multilayer ceramic capacitors. This is particularly the case where temperature coefficients of capacitance of +20, -30% from -25.degree. to 85.degree. C., corresponding to the 2D4 class of ceramic materials, are required.

Ceramic disc capacitors may be made by a process comprising the steps of forming a suitable ceramic dielectric composition, such as described above, to a required disc-like shape, firing the ceramic, providing opposite flat surfaces of the disc with a metal coating, soldering a respective connecting wire to each metal coating and encapsulating the body in an insulating material, such as a phenolic or epoxy resin.

The dielectric compositions described above may also be used for tubular ceramic capacitors which may be manufactured by a process comprising extruding a tubular ceramic body of rectangular cross-section, firing the ceramic body, depositing a metal layer on the inner and outer surfaces of the body, cutting the body up into required lengths and attaching connecting wires to the inner and outer metal layers prior to encapsulation in a phenolic or epoxy resin.

The dielectric compositions described above may also be useful for other capacitor applications, for example multilayer ceramic capacitors.