Method and device for measuring dielectric constant

Details Method and device for measuring dielectric constant Method and device for measuring dielectric constant

2001-06-07

2002-12-17

Oda, Christine (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Distributive type parameters

C324S632000

Reexamination Certificate

active

06496018

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method and a device for measuring dielectric constants by microwaves, of sheetlike substances such as a high-polymer sheet and paper including film, and three-dimensional articles such as moldings of plastic, resin, rubber and the like, as well as liquids such as an aqueous solution, a water dispersion liquid, an organic solvent liquid, liquid organic matter and the like.
TECHNICAL BACKGROUND
A dielectric constant is a physical value based on polarization in the inner part of a substance similar to a refractive index, and considered as an important physical value since the same is closely connected with electric, optical and dynamic physical properties. In a high-frequency domain such as light, the refractive index and the dielectric constant are in the relation:
(refractive index)
2
=dielectric constant
and, hence, it is also possible to make substitution by either one as the case may be. For a transparent substance, refractive index measurement by a refractive index meter is frequently employed. For a flat plate sample, various methods such as a method of obtaining the dielectric constant from the capacity of a plate capacitor, a method according to a microwave cavity resonator or a dielectric resonator and the like have been employed.
A method employing resonance of microwaves, utilizing such a principle that a resonance frequency shifts in correspondence to the dielectric constant, is directed to paper, a film, plastic, ceramic and rubber etc., and can be utilized regardless of presence/absence of optical transparency.
FIG. 1
illustrates a principle diagram of conventional dielectric constant measurement employing a microwave cavity resonator. It is a microwave resonator
6
comprising a microwave introduction part
2
on an end and a microwave sensing part
4
on another end while a portion between both end portions consists of a waveguide having a constant field vibrational direction. A slit
8
is provided on the resonator
6
in a direction perpendicularly crossing the axis of the resonator
6
on the position of a loop of standing waves. A sample
10
is arranged in the slit
8
and microwaves are introduced from the microwave introduction part
2
, for detecting the microwave intensity by the microwave sensing part
4
. The dielectric constant is measured from the amount of displacement between a resonance frequency without arranging the sample
10
in the slit
8
and a resonance frequency at the time of not arranging the sample (refer to Japanese Patent Publication Gazette No. 3-38632).
A method of measuring a dielectric constant with microwave dielectric resonators is shown in FIG.
2
.
FIG. 2
is a sectional view showing a conventional orientation measuring device employing dielectric resonators. Referring to the figure, it comprises a pair of dielectric resonators
12
a
and
12
b
opposed through a sample
10
, for making the dielectric resonators
12
a
and
12
b
generate field vectors having a single direction parallel to the surface of the sample
10
by a pair of antennas
14
a
and
14
b
oppositely arranged on side portions of the first dielectric resonator
12
a
through the dielectric resonator
12
a
and measuring the dielectric constant from the resonance characteristics thereof (refer to Japanese Utility Model Laying-Open Gazette No. 3-70368). Here, the antennas
14
a
and
14
b
are in the form of loops.
In the measuring device shown in
FIG. 1
or
FIG. 2
, the cavity resonator or the dielectric resonators hold the sample
10
and are oppositely arranged on both sides thereof, and hence the shape of the measured sample
10
is limited to a sheetlike one. However, in the recent plastic molding field, the necessity for measuring the dielectric constant of molded plastic or its anisotropy has become strong. For example, in a resin molding of an electrical appliance such as a PC and a television set, or a plastic vessel such as a PET bottle, the dielectric constant or its anisotropy remarkably varies with positions due to flowability distribution, pressure distribution or the like in molding, which has come into question. Therefore, measurement of the dielectric constant or its anisotropy of a three-dimensional article is in demand.
The refractive index of a liquid is currently measured by an optical method, and the sugar level of fruit juice, the degree of fatigue of oil, the concentration of soy sauce or the like may also be managed with this refractive index. However, with this method, there is a problem that it is difficult to measure an opaque liquid such as heavy oil that can hardly transmit light There is also such a problem that only a substance of which refractive index is up to 1.52 at the maximum can be measured by the optical method from a critical angle in total reflection of light
DISCLOSURE OF THE INVENTION
An object of the present invention is to make it possible to measure dielectric constants not only in a sheetlike sample but also in a sample such as a three-dimensional molding and a liquid sample.
An aspect of a dielectric constant measuring method according to the present invention includes the following steps:
(step 1) a step of arranging a sample measuring face of a single dielectric resonator arranged only on one side of a sample under a fixed condition on a standard sample of which dielectric constant is known, properly varying either one or both (referred to as “the dielectric constant and/or the thickness”) of the dielectric constant and the thickness of the standard sample for measuring a variance in the resonance frequency of the dielectric resonator with respect to each dielectric constant and/or thickness and acquiring a calibration curve in the variance of the resonance frequency responsive to the dielectric constant and/or the thickness.
(step 2) a step of measuring a variance of the resonance frequency by the dielectric resonator under the fixed condition as to a measured object sample of which the thickness is known.
(step 3) a step of obtaining the dielectric constant of the measured object sample from the measured value. and the calibration curve.
Another aspect of the dielectric constant measuring method is a dielectric constant measuring method arranging a sample measuring face of a single dielectric resonator arranged only on one side of a sample under a fixed condition on a measured object sample of which the thickness is. known, measuring a resonance frequency and obtaining the dielectric constant of the measured object sample according to the following equation (1):
&bgr;
gL
32 &pgr;/2
+P
&pgr;+tan
−1
(&agr;
2
/&bgr;g
)·tan h[tan h
−1
(&agr;
3
/&agr;
2
)+&agr;
2
L
2
]  (1)
&agr;
2
=(
k
c
2
−&ohgr;
0
2
&egr;
0
&mgr;
0
&egr;
s
)
½
&agr;
3
=(
k
c
2
−&ohgr;
0
2
&egr;
0
&mgr;
0
)
½
&bgr;
g
=(&ohgr;
0
2
&egr;
0
&mgr;
0
&egr;
r
−k
c
2
)
½
where &egr;
s
represents the dielectric constant of the sample, &egr;
r
represents the relative dielectric constant of the dielectric resonator, L represents the thickness of the dielectric resonator, &egr;
0
represents the dielectric constant of a measuring atmosphere. (air), &mgr;
0
represents the magnetic permeability of the measuring atmosphere, &ohgr;
0
represents a microwave resonance angular frequency, L
2
represents the thickness of the measured object sample, k
c
represents a constant (eigenvalue) determined by the shape of the dielectric resonator, an electromagnetic field mode or the like, and P represents 0, 1, 2, 3, . . . (these numerals mean integral times &lgr;
g
/2 in the axial direction).
Here, “a fixed condition” refers to operation of performing measurement while bringing the sample measuring face of the dielectric resonator into contact with the sample, or performing measurement while separating the sample measuring face of the dielectric resonator from the sample by a fixed distance.
In the dielectric constant measuring method according to the present invention, the resonance mode of the d

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