JPH0519970B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a variable capacitor, and, for example, to an element structure of a multilayer variable capacitor using a ferroelectric ceramic material for an insulating layer.

[Conventional technology]

Conventionally, variable capacitors have (a) multiple metal plates called variable capacitors that are arranged facing each other, and the effective area can be changed by moving one of the plates. (b) A device in which multiple ceramic capacitors, etc. are connected in parallel, each capacitor is connected to an operating IC, etc., and operated individually, and the capacitance value is changed using an electrical circuit.(c) As shown in Figure 6. As in Japanese Utility Model Application No. 58-172557, a part of the comb-shaped electrode 4a formed on the outermost ferroelectric layer 10 of a multilayer ceramic capacitor is mechanically cut by laser trimming or the like. , which changes the effective area between the internal electrodes 4b disposed facing each other with the ferroelectric layer 10 interposed therebetween;
There are other means such as

[Problem that the invention seeks to solve]

In the conventional variable capacitor (a) described above, the capacitor as a whole has a large shape and the capacitance is changed mechanically, so the capacitance cannot be changed by an electrical signal and requires an electrical response. It cannot be applied to circuits that

In (b), multiple capacitors are used in parallel, making it difficult to achieve high integration of the electric circuit, and since multiple capacitors are operated individually, the capacitance is output in steps.

In addition, (c) has the disadvantage that once a part of the electrode is cut off, the capacitance cannot be increased thereafter.

On the other hand, the present inventor has already proposed a variable capacitor characterized in that capacitance extraction electrodes are disposed at positions sandwiching bias extraction electrodes as shown in FIG. This variable capacitor is constructed by applying a DC voltage to the bias extraction electrodes 12a and 12b.
The capacitance between a and 11b can be changed linearly. However, in the variable capacitor shown in FIG. 5, no direct current bias is applied to the ferroelectric layers 10a and 10b sandwiched between the capacitance extraction electrodes 11a and 11b and the bias extraction electrodes 12a and 12b, so that the capacitance cannot be changed. The region where this occurs is the ferroelectric layer 10c sandwiched between the bias extraction electrodes 12a and 12b, and a large capacitance change cannot be obtained. For this reason, its use had to be limited to a narrow range.

[Means for solving problems]

An object of the present invention is to provide a variable capacitor that eliminates such conventional drawbacks.

According to the present invention, on the four opposing sides of a hexahedral element, the end faces of internal electrodes are exposed from an insulating layer using a ferroelectric material, and external electrodes are attached to the end faces of the internal electrodes. The variable capacitor is characterized in that one pair of the internal electrodes is used as capacitance extraction electrodes, and the other pair is used as bias extraction electrodes, and the bias extraction electrodes are arranged at positions where the capacitance extraction electrodes are sandwiched. can get.

Next, the principle of operation of the variable capacitor of the present invention will be explained.

Generally, ferroelectric materials used in capacitors etc. undergo a phase transition near the Curie point, resulting in a change in dielectric constant.
Abnormalities are observed in many physical properties such as elastic modulus and specific heat, but in particular, the dielectric constant ε suddenly increases at the Curie point. This is because the crystalline state of the ferroelectric material is extremely unstable near the Kyrie point, and almost no internal stress occurs. When a direct current bias is applied to the ferroelectric near this Curie point, internal stress is generated, and the dielectric constant ε becomes small.

As mentioned above, by intentionally controlling the external electric field of the ferroelectric near the Curie point, the dielectric constant ε
It is possible to change the voltage between the capacitance extraction electrodes (hereinafter referred to as capacitance electrodes) by applying a DC bias to the bias extraction electrodes (hereinafter referred to as bias electrodes) provided on the ferroelectric material near this Curie point. It becomes possible to change the capacity of

〔Example〕

The present invention will be explained below with reference to FIGS. 1 to 4.

FIG. 1 is an exploded perspective view of the variable capacitor of the present invention.

In the figure, reference numeral 1 denotes a green sheet 2 in which ferroelectric ceramic powder is kneaded with a synthetic resin binder such as polyvinyl alcohol (PVA), formed into a sheet by a method such as a doctor blade method, and cut into a desired shape. The internal electrode is made by applying a conductive paste to one side of a green sheet 1 and drying it.

Next, the laminated structure of each sheet of the capacitor of the present invention will be explained.

Reference numerals 11a and 11b denote an upper capacitor electrode and a lower capacitor electrode, which are exposed at the center with end surfaces 2a, which serve as lead-out electrodes of rectangular internal electrodes 2, aligned with one end surface of the front edge or rear edge of the green sheet 1.

12a and 12b are the aforementioned capacitor electrodes 11a and 11b.
An upper bias electrode and a lower bias electrode are exposed on the outside of the green sheet 1 with the end surfaces 2b of the rectangular internal electrodes 2 aligned with both end surfaces of the green sheet 1, and 13a and 13b are protective films without internal electrodes arranged on the outermost layer. This is a green sheet for use.

Next, the laminate of each sheet arranged in the configuration shown in Figure 1 is heat-processed from the top and bottom using a heat press, etc., to integrate them, and then main firing is performed in an electric furnace at a temperature of several hundred degrees to several thousand degrees. As shown in FIG. 3, external electrodes 21a are attached to the front and rear capacitance electrode end surfaces 2a, and external electrodes 21b are attached to the left and right bias electrode end surfaces 2b, thereby forming the variable capacitor of the present invention.

Next, a method for adjusting the capacitance of the variable capacitor of the present invention will be explained.

By applying a DC voltage to the external electrode 21b connected to the bias electrode, the bias electrode 1
The dielectric constant of the ferroelectric layer 10c sandwiched between the ferroelectric layer 2a and 12b changes to the negative side. FIG. 4 shows the change characteristics. When a DC bias voltage of 50 V is applied to a strong conductor with a thickness of 45 μm, the capacitance changes significantly by -70%.

As described above, the variable capacitor of the present invention can linearly change the capacitance between the capacitance electrodes by applying a DC voltage to the bias electrode using the electrodes as described above.

〔Effect of the invention〕

As explained above, the variable capacitor of the present invention has the following effects.

(i) Capacity can be obtained linearly over a wide range.

(ii) The capacitance can be changed by an external electrical signal.

(iii) Compact, highly reliable, and suitable for mass production.

The variable capacitor of the present invention can be used in a filter circuit, for example, to perform cut-off and pass operations at any frequency with a single chip.
It can be widely used in oscillation circuits, etc.

[Brief explanation of the drawing]

1, 2, and 3 are an exploded perspective view, a right sectional view, and a perspective view, respectively, of a variable capacitor according to an embodiment of the present invention, and FIG. 4 is a capacitance-bias voltage characteristic diagram of the variable capacitor of the present invention, FIG. 5 is a side sectional view of an embodiment of a variable capacitor previously proposed by the present inventor, and FIG. 6 is an exploded perspective view showing an example of a conventional variable capacitor. 1... Green sheet, 2... Internal electrode, 11
a, 11b...Capacitive electrode, 12a, 12b...Bias electrode, 2a...Capacitive electrode end surface, 2b...Bias electrode end surface, 21a, 21b...External electrode,
10... Ferroelectric layer, 10a, 10b... Capacitive electrode, ferroelectric layer between bias electrodes, 10c... Ferroelectric layer between bias electrodes, 4a, 4b... Capacitive electrode.

Claims (1)

[Claims] 1. Exposing the end surfaces of the internal electrodes on the four opposing sides of the hexahedral element from the insulator layer using a ferroelectric material,
In a variable capacitor in which an external electrode is attached to an end face of an internal electrode, one pair of the internal electrodes is used as a capacitance extraction electrode, the other pair is used as a bias extraction electrode, and the bias extraction electrode is a capacitance extraction electrode. A variable capacitor characterized by being placed in a position where it is sandwiched between.