Multi layer Smectite Capacitor (MLSC)

Multilayer smectite capacitor (MLSC) provided with a new dielectric film, and a first electric and second electrode formed sandwiching it and facing each other, wherein has dielectric film. The dielectric film in this capacitor is smectite, and either or both of said first electrode and second electrode contain at least one metal selected from group consisting of Cu, Ni. Thin internal conductors are each placed between the smectite dielectric layer and arranged in parallel. The smectite dielectric layers have a thickness of about 0.6 to less than 2 micrometer. The internal conductors have a thickness of about 0.2 to 0.4 micrometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment of the present invention will now be described in detail.FIG. 1is a sectional view showing a Multilayer Smectite Capacitor according to an embodiment of the present invention. The multilayer smectite capacitor includes a plurality of stacked dielectric layers4and thin internal conductors6that are each placed between the dielectric layers4and arranged in parallel. The dielectric layer4and internal conductors6have been prepared by a co-firing process and from a smectite body5.

External conductors (nickel, copper and silver)3aand3bare each placed at both ends of the smectite body5. First metal (nickel) coatings2aand2bare placed on the external conductors3aand3b, respectively, and second metal (tin) coating1aand1bare placed on the first metal coatings2aand2b, respectively.

The internal conductors6are arranged in parallel in the thickness direction on the ceramic body. The internal conductors6are classified in to the first conductors6a,6d,6f,6h,6j, and6land second internal conductors6b,6c,6e,6g,6i, and6k. One of each of the first internal conductors6a,6d,6f,6h,6j, and6lis electrically connected to the external conductors3aand one of each second internal conductors6b,6c,6e,6g,6i, and6kis electrically connected to external conductor3b, whereby static capacitors are formed between the first internal conductors6a,6d,6f,6h,6j, and6land second conductors6b,6c,6e,6g,6i, and6k, respectively.

In this embodiment layer4has a thickness of about 0.6 to less than 2 micrometer. And internal conductors6have a thickness of about 0.2 or more to 0.4 or less micrometer. The internal conductors6each have spaces therein/there between. The total area percentage of the space in each internal conductor6is between 20 to 40 percent of the area of the internal conductors6.

The reasons why the thickness of the dielectric layer4and internal conductors6and the area percentage of space are limited to the above range will now be described in detail. When the internal conductors6are caused to partially shrink such as that the spaces are formed in the internal conductors6, the additive containing glass component principally containing Si is selectivity deposited in the spaces. The additive can be prevented from being segregated at grain boundaries or interface between the dielectric layers4and internal conductors6, and capacitance and reliability are enhanced although the internal conductors6have a decrease area.

The dielectric layers4preferably have a small thickness. When the dielectric layer4have a thickness about or more, the capacitance thereof is too small to obtain MLSC having small size and high capacitance even if the dielectric layer4have large dielectric constant. In contrast, it is difficult to form dielectric layers that have thickness of less than 0.2 micrometer because of technical limitation. Thus, in the present invention the thickness of the smectite layer4is within of 0.2 to 2 micrometer.

When internal conductors have a thickness of less than 0.2 micrometer, the internal conductors partially shrink and therefore have a decrease area even if the internal are fired at a temperature lower that the melting point of a conductive material contained in the internal conductors. Therefore, the area of overlapping regions of the internal conductors facing each other is insufficient to obtain a desired capacitance. In contrast, when the internal conductors have a thickness of more than 0.4 micrometer, structure defects such as de laminations and cracks may occur due to increase in war page of layered body including stacked dielectric layer and internal conductors in common with printed electrode having a large thickness.

Thus In the present invention, the thickness of the internal conductors6is within 0.2 to 0.4 micrometer. The internal conductors6can be prevented from partially shrinking by using another sintering additive that is effective in sintering a smectite dielectric material at a lower temperature or by sintering a smectite dielectric material at temperature lower than the melting point of a metal component contained in the internal conductors6. However, the total area percentage of the spaces in each internal conductors6is reduced to 20%. In contrast, when the total area percentage of the space in each two is 40% or more, the area of overlapping region of the internal conductors6facing each other is insufficient, hence, MLSC has an insufficient capacitance. Thus, in the present invention the total area parentage of the spaces in each internal conductor6is within a range 20 to 40%.

A process for producing the MLSC will now be described. The smectite powder contains the structural units of that can be derived from the structures of pyrophyllite and talc. Unlike pyrophyllite and talc, the 2:1 silicate layers of smectite have a slight negative charge owing to ionic substitutions in the octahedral and tetrahedral sheets. The net charge deficiency is normally smaller than that of vermiculite from 0.2 to 0.6 per O10(OH)2and is balanced by the interlayer cations as in vermiculite. This weak bond offers excellent cleavage between the layers.

The distinguishing feature of the smectite structure is that water and other polar molecules (in the form of certain organic substances) can, by entering between the unit layers, cause the structure to expand in the direction normal to the basal plane. Thus this dimension may vary from about 9.6 Å, when there are no polar molecules between the unit layers, to nearly complete separation of the individual layers.

The structural formula of smectites of the dioctahedral aluminous species may be represented by (Al2yMg2+/y)(Si4−xAlx)O10(OH)2M+/x+y·nH2O, where M+is the interlayer exchangeable cation expressed as a monovalent cation and where x and y are the amounts of tetrahedral and octahedral substitutions, respectively (0.2≦x+y≦0.6). The smectites with y>x are called montmorillonite and those with x>y are known as beidellite. In the latter type of smectites, those in which ferric iron is a dominant cation in the octahedral sheet instead of aluminum and magnesium are called nontronite.

Although less frequent, chromium (Cr3+) and vanadium (V3+) also are found as dominant cations in the octahedral sheets of the beidellite structure, and chromium species are called volkonskoite. The ideal structural formula of trioctahedral ferromagnesian smectites, the series saponite through iron saponite, is given by (Mg,Fe2+)3(Si4−xAlx)O10(OH)2M+/x·nH2O.

The tetrahedral substitution is responsible for the net charge deficiency in the smectite minerals of this series. Besides magnesium and ferrous iron, zinc, cobalt, and manganese are known to be dominant cations in the octahedral sheet. Zinc dominant species are called sauconite.

There are other types of trioctahedral smectites in which the net charge deficiency arises largely from the imbalanced charge due to ionic substitution or a small number of cation vacancies in the octahedral sheets or both conditions. Ideally x is zero, but most often it is less than 0.15. Thus, the octahedral composition varies to maintain similar amounts of the net charge deficiency as those of other smectites. Typical examples are (Mg3−y Liy) for stevensite and hectorite, respectively.

According to the table below, be discovered that smectite mineral have higher dielectric constant than other material such as Mica, Polyester, Teflon and etc. Typical values for dielectric constant are as follows.

Increase in dielectric permittivity in Smectite Clay is related to: (a) a significantly high imaginary part of the relative permittivity; and (b) a frequency-dependent response (called dielectric dispersion). For smectites, a significant part of water is located between layers inside tactoids in intra-domain pores. Within tactoids or quasi-crystals, the corresponding interlayer spacing is either 18.6 A° for Ca-smectites, or 35 to 100 A° for Na.

However, clay minerals have been shown to exhibit dielectric dispersion to differing extents. Smectites also cause signal attenuation and dispersion, resulting in smaller signal amplitude and longer rise time. This means that the real (∈′) and imaginary (∈″) parts of the permittivity describing energy storage and energy losses respectively, change as a function of frequency. The smectites cause dispersion of the reflected signal, resulting in longer rise time, and evidence showed that there is a rise time related measurement error.

In general, Kaolinite and mica exhibits the lowest dispersion, smectites show the highest with Illite being somewhat intermediate. Therefore, according experimental results we obtain that smectite clay have dielectric constant value between 8.5-10.5.

In the MLSC of this embodiment, the dialectic layers4have a thickness of less than 2 micrometers describe above. In order to prepare the dielectric layer4with a small thickness, the average of particle size of the smectite powder is preferably fine and uniform. When the smectite powder had an average particle size of less than 100 nm, the smectite powder violently reacts with the additive, hence, the fired dielectric layer4have a large particle size, which causes deterioration in temperature coefficient and voltage coefficient of capacitance. In contrast, when the smectite powder has an average particle size more than 250 nm, the smectite powder has low reactivity with additive and cannot therefore be sintered at a low temperature.

Furthermore, the spaces in each internal conductor6have an excessively large area, which causes a decrease in capacitance and deterioration in electrical property. Therefore, MLSC with high reliability cannot obtain. Thus, the smectite powder preferably has an average particle size of 100 to 250 nm.

Next, the following additives are prepared, a sintering additive containing SiO2, compound, a compound additive containing a rare-earth element, Ba, Zr, Mn, Mg, Si, B, Al, or Li. These additively are uniformly mixed with the smectite powder dispersed in an organic solvent, and the mixture is dried and then heated such that the organic solvent is removed from the mixture, whereby smectite ingredient powder is prepared. A predetermined amount of a binder, plasticizer, and organic solvent are mixed with the smectite ingredient powder in a ball mil by a wet process, whereby smectite slurry is prepared. The smectite slurry is then formed in to smectite sheets by a known method that shown above. On the other hand Metal films for forming the internal conductors6are prepared by a thin film forming process such as electro plating process.

The resulting metal layer is patterned using a resist material, whereby the metal films are formed on the PET Films. Since the internal conductors6are prepared using the metal films, a difference in thickness between the following portion of a layered body including the smectite sheets and the metal films can be reduced, a portion having the metal films and another portion having no metal filmed. Therefore, structural defects can be prevented from occurring in the layered body if the layered body includes a large number of layers. In order to manufacture the MLSC at low cost, a base metal such as nickel, copper, and tin is preferably used for forming the metal films.

The total area percentage of the spaces in each internal conductor6can be controlled by varying the thickness of the internal conductor6or changing a material for the metal films. The thickness of the internal conductor6can be ready controlled by preparing the internal conductor6by a thin films-forming process. The metal films preferably have such roughness of 50-60 nm. The metal films having such a surface roughness are usual for preparing elements having high reliability when the smectite sheets have a small thickness.

A large number of the smectite sheets each having the corresponding internal conductors6are stacked such that connection of the internal conductors6extending outside are alternately arranged, whereby the layered body is prepared. The binder is removed from the layered body, and the resulting layered body if firmed under an oxygen partial pressure of 10-9 MPa to 10-12 MPa in a reductive atmosphere containing H2, whereby the sintered smectite body5is prepared.

The resulting sintered smectite body5is then fired, whereby the external conductors, that are copper, nickel, and silver,3aand3bare each formed on the corresponding end faces. A conductive material contained in the internal conductors6or external conductors3aand3bis not particularly limited. The internal conductors6and external conductors3aand3bmay contain the same material. The first containing, that is nickel,2aand2bare formed on the external conductors3aand3b, respectively, and the second metal coating, that is tin,1aand1bare formed on the first metal coating2aand2b, respectively, by an electroplating process, whereby the MLSC is obtained (FIG. 2).

As described above. In this embodiment, the thickness of the dielectric layer4is limited to a predetermined range; the thickness of the internal conductors6is limited to a predetermined range. Therefore, the MLSC that has high reliability and a large capacity and includes a large number of thin layers can be obtained.

It is understood that the above description and drawings are illustrative of the present invention and that changes may be made in as it is best known in the art without departing from the scope of the present invention as defined in the following claims.