Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2003-04-01
2004-07-13
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S311000, C336S040000, C029S025410
Reexamination Certificate
active
06762925
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramic electronic components and methods for making the same. In particular, the present invention relates to a ceramic electronic component having superior impedance characteristics in high-frequency bands such as the gigahertz band and exhibiting high impedance over a broad frequency range, and relates to a method for making the same.
2. Description of the Related Art
With use of electronic devices in high-frequency ranges in recent years, inductors, LC composite electronic components, LR composite electronic components, and LCR composite electronic components that can be used in the gigahertz band are required.
Unfortunately, stray capacitance generated parallel to a coil in inductors for high-frequency bands significantly affects the impedance of the coil. In particular, a minute stray capacitance in the range of 1/100 pF to 1/10 pF significantly affects the impedance in the gigahertz band. If the stray capacitance is reduced while required characteristics are maintained, the dielectric constant ∈ of the ferrite used as a magnetic material should be reduced. However, it is virtually impossible for the dielectric constant ∈ of the ferrite to be reduced to, for example, 13 to 14, because of the structural factor of the ferrite.
A possible way to reduce the dielectric constant ∈ is a composite magnetic material that is prepared by compounding of a material having a low dielectric constant, such as resin or glass, with a magnetic material. In such composite magnetic material, the magnetic particles are covered by the resin or glass nonmagnetic material. Since the magnetic path is segmented, the permeability of the composite magnetic material is extremely low.
A porous ferrite sintered compact having a porosity of 20% to 70% is known as a ferrite material having a low dielectric constant which is used in an electromagnetic wave absorber, as disclosed in Japanese Unexamined Patent Application Publication No. 55-526300. Since this ferrite sintered compact containing a high rate of pores has a low dielectric constant and a continuous magnetic path, it does not cause a discontinuous and large variation in electromagnetic characteristics. In the porous ferrite sintered compact, individual ferrite particles are magnetically coupled with each other even at a high porosity; hence, this ferrite sintered compact exhibits a small dependence of complex permeability on frequency dispersion characteristics. However, a high porosity of this porous ferrite sintered compact causes decreased mechanical strength, and more particularly, low flexural strength, of a resulting electronic component.
A porous ceramic electronic component is disclosed in Japanese Unexamined Patent Application Publication No. 11-67575. This ceramic electronic component is composed of ceramic containing 3% to 30% by volume of pores with a diameter of 1 &mgr;m to 3 &mgr;m and an internal electrode provided in the ceramic. Such a porous ceramic electronic component has a low dielectric constant and thus exhibits improved impedance characteristics.
In this technology, the upper limit of the pore content is 30% by volume because a pore content exceeding the upper limit decreases the flexural strength of the ceramic. Thus, the relative dielectric constant cannot be reduced to a level that is required for current ceramic electronic components having superior characteristics. Furthermore, the pores in the ceramic of this ceramic electronic component easily absorb moisture. A high water absorbance deteriorates reliability of the component.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a ceramic electronic component including a ceramic sintered compact having a low dielectric constant and high mechanical strength and a method for making the ceramic electronic component.
According to a first aspect of the present invention, a ceramic electronic component comprises a ceramic sintered compact containing about 35 to 80 volume percent pores; and an electrode provided inside the ceramic sintered compact, wherein the pores are fully or partly filled with resin or glass.
The ceramic electronic component contains about 35 to 80 volume percent pores that are fully or partly filled with the resin or glass. Furthermore, the ceramic phase of the ceramic sintered compact is continuous in the present invention. Accordingly, the ceramic electronic component exhibits a decreased dielectric constant without deterioration of electrical characteristics and mechanical strength of the ceramic sintered compact.
The present invention is based on the recognition by the inventors that a small amount of resin or glass packed in the pores significantly improves the tensile stress of ceramic materials have high compression stress but low tensile stress, such as ferrite. Thus, a ceramic electronic component having a porosity of about 80 volume percent according to the present invention favorably compares with a conventional component having a porosity of 30 volume percent in that the dielectric constant is drastically decreased to about 6 or less without deterioration of the mechanical strength such as flexural strength and electrical characteristics.
The ceramic electronic component of the present invention has pores in the ceramic sintered compact. If the ceramic sintered compact is magnetic, the permeability of the porous magnetic sintered compact decreases to some extent compared with the corresponding solid compact. However, the magnetic ceramic sintered compact has a continuous magnetic path. Thus, the permeability of the magnetic material is maintained in the sintered compact. This indicates that the cross-point frequency at which &mgr;′=&mgr;″ does not substantially change.
Preferably, the average pore diameter is in the range of about 5 &mgr;m to 20 &mgr;m. A diameter of less than about 5 &mgr;m causes the formation of closed pores that cannot be impregnated with glass or resin. A diameter exceeding about 20 &mgr;m causes a noticeable reduction in strength of the sintered compact. More preferably, the average pore diameter is in the range of about 5 &mgr;m to 10 &mgr;m.
The porosity (volume rate) of the pores must be at least about 35 volume percent in order that the ceramic sintered compact has a sufficiently low dielectric constant with satisfactory mechanical strength. However, a porosity exceeding about 80 volume percent causes a noticeable decrease in mechanical strength that inhibits subsequent processing such as resin or glass impregnation. Thus, the upper limit of the porosity is about 80 volume percent in the present invention. Preferably, the porosity is in the range of about 40 volume percent to about 50 volume percent. Since the resin or glass packed in the pores reinforces the ceramic sintered compact, the ceramic sintered compact can be processed within the pore diameter range of about 5 &mgr;m to 20 &mgr;m and the porosity of about 80 volume percent or less.
Preferably, the ceramic sintered compact is formed by firing a green compact comprising a ceramic raw material, a binder, and a spherical or granular combustible material having adhesiveness to the binder. If a magnetic sintered compact is formed in this process, the porous ceramic sintered compact has a continuous magnetic path. Thus, the ceramic electronic component has desired magnetic characteristics, reduced stray capacitance, and required electrical and mechanical characteristics.
Preferably, the resin or glass in the pores contains internal pores. The internal pores contribute to a further decrease in dielectric constant of the ceramic sintered compact.
Preferably, the ceramic sintered compact is a magnetic ceramic sintered compact. The magnetic ceramic material can be used for production of inductors. For example, an inductor as a ceramic electronic component comprising the magnetic ceramic sintered compact has desired magnetic characteristics, reduced stray capacitance due to a decreased dielectric constant, and required elec
Kawabata Toshio
Nishii Motoi
Otsuki Takehiko
Sakamoto Yukio
Sugitani Masami
Dickstein Shapiro Morin & Oshinsky LLP.
Dinkins Anthony
Murata Manufacturing Co. Ltd.
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