JP2012069580A - Dielectric ceramic for tunable devices - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide dielectric ceramic for tunable devices which has large variation of a relative dielectric constant for dc bias electric fields, small temperature dependence of the relative dielectric constant, and small dielectric loss in a wide frequency range, and enables low-temperature sintering.SOLUTION: The dielectric ceramic for tunable devices has a composition represented by the general formula: (1-x-y)NaNbO-xSrTiO-yCaTiO, wherein 0≤x<1, 0≤y<1 and 0<x+y<1. Furthermore, x and y preferably satisfy 0≤x≤0.2, 0≤y≤0.2, and 0<x+y≤0.2, and are more preferably within the quadrangle whose vertices are (x, y)=(0, 0.15), (0.09, 0.06), (0.04, 0.06), (0, 0.10), or on edges of the quadrangle.

Description

  The present invention relates to dielectric ceramics for tunable devices, and more particularly, to dielectric ceramics for tunable devices used in various tunable devices that actively use changes in relative dielectric constant due to a DC bias electric field for signal processing. .

A dielectric is a substance that generates dielectric polarization when an electric field is applied. Dielectric materials are used for capacitors, memories, filters, antennas, resonators, and the like. In general, the dielectric is required to have a dielectric constant having a magnitude corresponding to the application, a low dielectric loss, a temperature dependence of a low resonance frequency, a temperature dependence of a low dielectric constant, and the like. In addition, the dielectric material may be required to have a low relative dielectric constant and an applied electric field dependency in certain applications (for example, switching applications).
On the other hand, titanium in which strontium titanate (SrTiO ₃ ), which is a paraelectric material, is dissolved in barium titanate (BaTiO ₃ ) and lead titanate (PbTiO ₃ ), which are ferroelectric materials having a perovskite crystal structure. For barium strontium oxide ((Ba, Sr) TiO ₃ ) and lead strontium titanate ((Pb, Sr) TiO ₃ ), nonlinear dielectric constant changes under a DC bias electric field have been reported. In addition, a tunable device using a characteristic (tunable characteristic) that can change the relative dielectric constant by an applied electric field has been proposed.

Various proposals have heretofore been made regarding the composition of such various dielectrics and their applications.
For example, in Patent Document 1, aATiO ₃ —bNaNbO ₃ (A is one or two of Ca and Sr) is represented by 0.03 ≦ a ≦ 0.170, 0.83 ≦ b ≦ 0. 970, a + b = 1, and a dielectric ceramic composition in which an ATiO ₃ component and a NaNbO ₃ component form a solid solution is disclosed.
In this document, by setting the composition range to the above range, the dielectric constant can be adjusted within the range of 170 to 1400, and the temperature coefficient τf of the resonance frequency can be adjusted within the range of −650 to −1800 ppm / K. Are listed.

In Patent Document 2, a capacitor element having a well region formed on the entire boundary surface of a dielectric layer and a capacitor element having a dielectric layer in which no well region is formed are connected in parallel by ion implantation. A thin film laminated device is disclosed.
In the same document,
(1) The capacitor element in which the well region is not formed has a maximum capacitance of zero V, whereas in the capacitor element in which the well region is formed, the CV curve is shifted in the plus direction with respect to the voltage axis. Point and
(2) When these capacitor elements are connected in parallel, the maximum value of the capacitance shifts in the positive direction with respect to the voltage axis, and the width of the CV curve increases, and a stable capacitance can be obtained in a wide voltage range (ie, The voltage dependency of capacitance is small).

Non-Patent Document 1 reports a tunable device using a (Bi, Zn) ₂ (Zn, Nb) ₂ O ₇ -based pyrochlore type dielectric.
Furthermore, Non-Patent Documents 2 to 5 report dielectric characteristics in the kHz band of NaNbO ₃ —SrTiO ₃ and NaNbO ₃ —CaTiO ₃ under no-bias electric field.

Tunable devices used in microwaves and millimeter waves are desired to have a high Q · f value, a large change in relative permittivity with respect to a DC bias electric field, and stability of the relative permittivity with respect to temperature.
Here, the Q · f value is a value obtained by multiplying the reciprocal of the dielectric loss by the resonance frequency, and the higher the Q · f value, the lower the dielectric loss.

(Ba, Sr) TiO ₃ and (Pb, Sr) TiO ₃ exhibit a large change in relative permittivity with respect to the DC bias electric field. On the other hand, these materials also show a large value of the temperature dependence of the dielectric constant. Therefore, in these materials, in order to improve the temperature dependence of the relative dielectric constant, a low dielectric constant compound (for example, MgO, Al ₂ O ₃ etc.) having a small absolute value of the temperature coefficient is added. However, this method has a problem that the amount of change in the relative dielectric constant with respect to the DC bias electric field is also greatly reduced.
Moreover, when producing a sintered body of (Ba, Sr) TiO ₃ or (Pb, Sr) TiO _{3 by} a solid phase reaction process, a sintering temperature of about 1400 ° C. is required. Furthermore, (Pb, Sr) TiO ₃ is not environmentally preferable because it contains lead as a raw material.

Ag (Nb, Ta) O ₃ , which is a kind of perovskite compound, has been proposed as a tunable dielectric exhibiting a relatively high Q · f value, but the relative permittivity change with respect to the DC bias electric field is relatively small. . Moreover, since this material uses a noble metal element as a raw material, there is a problem that the manufacturing cost is high.
On the other hand, certain pyrochlore type compounds are also known to exhibit tunable properties. However, the pyrochlore type compound has a problem that the relative permittivity change with respect to the bias electric field is smaller than that of the perovskite type compound.

By the way, a ferroelectric shows a high relative permittivity change with respect to a DC bias electric field. (Ba, Sr) TiO ₃ and (Pb, Sr) TiO ₃ based tunable devices utilize high tunable characteristics exhibited by ferroelectric phases such as BaTiO ₃ and PbTiO ₃ .
On the other hand, as a phase state in which free energy is close to that of a ferroelectric material, an antiferroelectric material is cited. Antiferroelectric materials are also expected to exhibit a high relative dielectric constant change with respect to the bias electric field by dissolving a paraelectric material in the same manner as ferroelectric materials. However, there has never been proposed an example of dielectric ceramics for tunable devices based on NaNbO ₃ which is an antiferroelectric material.

JP 2009-234888 A JP 2004-031408 A

Thin Solid Films, 419, 183-188 (2002) J. Am. Ceram. Soc., 43, 348-353 (1960) Chem. Mater., 16, 5370-5376 (2006) J. Am. Ceram. Soc., 90, 2122-2127 (2007) Bulltein of the Academy of Science of the U.S.S.R.Physical Series, 22, 1486-1489 (1058)

The problem to be solved by the present invention is that the relative permittivity change with respect to the DC bias electric field is large, the temperature dependence of the relative permittivity is small, the dielectric loss in a wide frequency range is small, and tunable capable of low-temperature sintering. It is to provide dielectric ceramics for devices.
Another object of the present invention is to provide a dielectric ceramic for a tunable device that does not contain an environmental load element and is low in cost.

In order to solve the above problems, the dielectric ceramics for a tunable device according to the present invention has a composition represented by the formula (1).
(1-x-y) NaNbO 3 -xSrTiO 3 -yCaTiO 3 ··· (1)
However, 0 ≦ x <1, 0 ≦ y <1, 0 <x + y <1.
x and y are preferably 0 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2, and 0 <x + y ≦ 0.2.

When the solid solution of SrTiO ₃ and / or CaTiO ₃ is a paraelectric in NaNbO ₃ is anti-ferroelectric dielectric loss (or, Q · f value), the dielectric constant, the dielectric constant for the DC bias field Any one or more of the change rate, the temperature dependency of the relative permittivity, and the frequency dependency of the relative permittivity is improved.
In addition, the dielectric ceramics for tunable devices according to the present invention based on the antiferroelectric material, unlike the dielectric ceramics based on the ferroelectric material, have a low dielectric loss over a wide range of frequencies including the high frequency range. Indicates.
Further, NaNbO ₃ is a hardly sinterable material, but when SrTiO ₃ and / or CaTiO ₃ is dissolved therein, the sintering temperature can be lowered. In addition, it is not necessary to add an environmental load element or an expensive noble metal element in order to obtain the above-described characteristics.

It is a diagram showing a preferred composition of (1-x-y) NaNbO 3 -xSrTiO 3 -yCaTiO 3. FIG. 2A is a cross-sectional view showing a first specific example of a capacitive element using dielectric ceramics. FIG. 2B is a plan view showing a second specific example of a capacitive element using dielectric ceramics. FIG. 2C is a diagram illustrating an example of a circuit of the tunable device.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Dielectric ceramics for tunable devices]
[1.1. composition]
The dielectric ceramics for tunable devices according to the present invention (hereinafter also simply referred to as “dielectric ceramics”) has a composition represented by the formula (1).
(1-x-y) NaNbO 3 -xSrTiO 3 -yCaTiO 3 ··· (1)
However, 0 ≦ x <1, 0 ≦ y <1, 0 <x + y <1.

In the formula (1), x represents the molar fraction of SrTiO ₃ contained in the dielectric ceramic. y represents the molar fraction of CaTiO ₃ contained in the dielectric ceramic. The dielectric ceramics may contain either SrTiO ₃ or CaTiO ₃ , or may contain both.

Among formulas (1), x and y preferably satisfy the relationship of formula (1.1).
0 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2, 0 <x + y ≦ 0.2 (1.1)
The dielectric ceramic satisfying the formula (1.1) has a dielectric loss (or Q · f value), a relative permittivity, a change rate of the relative permittivity with respect to a DC bias electric field, a temperature stability of the relative permittivity, and a ratio. It shows excellent characteristics with respect to any one or more of the frequency dependence of the dielectric constant.
Specifically, the dielectric ceramic satisfying the formula (1.1) is
(A) The absolute value of the relative dielectric constant voltage change rate described later is 0.3% or more.
(B) The frequency dependence of the dielectric constant is small from the kHz band to the GHz band.
(C) low dielectric loss (high Q · f value) in the high frequency range,
(D) There is an advantage that dense sintering is possible at a sintering temperature of 1300 ° C. or less.

x and y are 4 in particular, (x, y) = (0, 0.15), (0.09, 0.06), (0.04, 0.06), (0, 0.10). It is preferable to be within an area surrounded by points (including on the boundary line) (see FIG. 1).
Dielectric ceramics in which x and y are in the hatched region shown in FIG. 1 exhibit a relative dielectric constant voltage change rate of 1% or more. The relative dielectric constant voltage change rate will be described later.

x and y are further in a region surrounded by three points of (x, y) = (0.09, 0.06), (0.04, 0.06), (0.05, 0.10). (Including on the boundary line).
Dielectric ceramics in which x and y are in such a region have the advantage of simultaneously exhibiting a relative dielectric constant voltage change rate of 1.0% or more and a Q · f value of 100 GHz or more.

[1.2. Crystal structure]
NaNbO ₃ is an antiferroelectric material having a perovskite crystal structure. On the other hand, SrTiO ₃ and CaTiO ₃ are both paraelectric materials having a perovskite crystal structure.
Dielectric ceramics according to the present invention has a solid solution of SrTiO ₃ and / or CaTiO ₃ a predetermined amount of NaNbO _3, regardless of the x and y, take a perovskite-type crystal structure.

[1.3. Dielectric properties]
[1.3.1. Dielectric constant]
The dielectric constant of the dielectric ceramic depends on x and y, and can be controlled in the range of about 150 to 2100 depending on the values of x and y. In general, as the relative permittivity increases, the absolute amount of change in relative permittivity with respect to the DC bias voltage increases, but at the same time, the dielectric loss also increases. Therefore, it is preferable to select the relative dielectric constant so that the balance with other dielectric characteristics is the best.

[1.3.2. Relative permittivity voltage change rate (tunability)]
“Relative permittivity voltage change rate” refers to the rate of change of relative permittivity with respect to the DC bias electric field, and specifically refers to the value expressed by equation (2).
Relative permittivity voltage change rate (%) = (εr ₀ −εr ₁ ) × 100 / εr ₀ (2)
Where εr ₀ is the relative dielectric constant at a DC bias electric field of ₀ kV / cm,
εr ₁ is a relative dielectric constant at a DC bias electric field of 10 kV / cm.

The dielectric ceramic according to the present invention has tunable characteristics. In the present invention, “tunable characteristic” means that the absolute value of the relative dielectric constant voltage change rate represented by the equation (2) is 0.1% or more under the conditions of room temperature and frequency of 100 kHz. Generally, as the absolute value of the relative permittivity voltage change rate increases, the absolute value of the change amount of the relative permittivity increases, so that the change in relative permittivity can be easily used for signal processing.
The relative dielectric constant voltage change rate depends on the values of x and y. When x and y are optimized, a dielectric ceramic having an absolute value of a relative dielectric constant voltage change rate of 0.3% or more at room temperature and a frequency of 100 KHz can be obtained. When x and y are further optimized, a dielectric ceramic having an absolute value of a relative dielectric constant voltage change rate of 0.5% or more, 0.7% or more, 0.9% or more, or 1.5% or more is obtained. can get.

[1.3.3. Q · f value (dielectric loss)]
The Q · f value is a value obtained by multiplying the reciprocal of the dielectric loss by the resonance frequency, and the higher the Q · f value, the lower the dielectric loss.
The Q · f value depends on the values of x and y. When x and y are optimized, a dielectric ceramic having a Q · f value of 50 GHz or more is obtained. When x and y are further optimized, a dielectric ceramic having a Q · f value of 100 GHz or more is obtained.

[1.3.4. Relative permittivity frequency change rate]
The “relative dielectric constant frequency change rate” refers to the change rate of the relative dielectric constant with respect to the frequency of the high frequency current applied to the dielectric ceramic, and specifically refers to the value represented by the equation (3).
Specific dielectric constant frequency change rate (%) = (εr ₂ −εr ₃ ) × 100 / εr ₂ (3)
Where εr ₂ is the relative dielectric constant at a frequency of 100 kHz,
εr ₃ is a relative dielectric constant at a frequency of 1 GHz.

In general, the smaller the absolute value of the relative dielectric constant frequency change rate, the more advantageous it is that it can be used as a circuit element for a wide range of frequencies.
The relative permittivity frequency change rate depends on the values of x and y. When x and y are optimized, a dielectric ceramic having a relatively small absolute value of the relative dielectric constant frequency change rate can be obtained under conditions of room temperature and no bias. The smaller the absolute value of the relative dielectric constant frequency change rate, the better.

[1.3.5. Relative permittivity temperature change rate]
“Relative permittivity temperature change rate” refers to the rate of change of relative permittivity with respect to temperature, and specifically refers to the value represented by equation (4).
Specific dielectric constant temperature change rate (%) = (εr ₅ −εr ₄ ) × 100 / εr ₄ (4)
Where εr ₄ is the relative dielectric constant at a temperature of 20 ° C.
εr ₅ is a relative dielectric constant at a temperature of 80 ° C.

In general, the smaller the absolute value of the relative dielectric constant temperature change rate, the more advantageous it is that it can be used as a circuit element used in a wide temperature range.
The relative dielectric constant temperature change rate depends on the values of x and y. When x and y are optimized, a dielectric ceramic having an absolute value of a relative dielectric constant temperature change rate of 50% or less is obtained under the condition of a frequency of 100 kHz and no bias. When x and y are further optimized, a dielectric ceramic having an absolute value of a relative dielectric constant temperature change rate of 40% or less, 30% or less, or 20% or less is obtained.

[1.4. Sinterability]
“Sintering temperature” refers to the lowest temperature at which a relative density of 98% or more is obtained by sintering for 1 hour.
NaNbO ₃ , SrTiO ₃ and CaTiO ₃ are all hard-to-sinter materials having a sintering temperature exceeding 1300 ° C. alone. In contrast, when the solid solution of SrTiO ₃ and / or CaTiO ₃ a predetermined amount of NaNbO _3, it is possible to lower the sintering temperature.
The sintering temperature depends on the values of x and y. When x and y are optimized, a dielectric ceramic having a sintering temperature of 1300 ° C. or lower is obtained.

[2. Tunable device]
The dielectric ceramic according to the present invention is used for a tunable device.
A “tunable device” refers to a device that positively uses a change in relative dielectric constant due to a DC bias electric field of a dielectric for signal processing.
Specific examples of the tunable device include a tunable antenna, a multiband pass filter, a phase shifter, and a variable capacitance element.

  FIG. 2A shows a first specific example (cross-sectional view) of a capacitive element using dielectric ceramics. In FIG. 2A, the capacitive element 10 is obtained by forming a lower electrode layer 14, a dielectric layer 16, and an upper electrode layer 18 in this order on a substrate 12. The dielectric ceramic according to the present invention is used for the dielectric layer 16.

  FIG. 2B shows a second specific example (plan view) of a capacitive element using dielectric ceramics. In FIG. 2B, the capacitive element 20 has a dielectric layer 22 formed between metal lines 24 and 26 formed on a substrate (not shown). The dielectric ceramic according to the present invention is used for the dielectric layer 22. In this case, the metal lines 24 and 26 on both sides of the dielectric layer 22 serve as electrodes.

  FIG. 2C shows an example of a circuit of a tunable device using such a capacitive element. 2C, the tunable device 30 includes a capacitive element 32, an inductance 34, and a variable DC power source 36. The dielectric ceramic according to the present invention is used for the dielectric layer of the capacitive element 32. An input terminal is connected to one electrode of the capacitive element 32, and an output terminal is connected to the other electrode. In addition, a variable DC power source 36 is connected to the capacitive element 32 via an inductance 34.

  When a high frequency signal is input to the tunable device 30 from the input terminal, the high frequency signal is output from the output terminal. At this time, when the voltage applied to the capacitive element 32 is adjusted using the variable DC power source 36, the relative dielectric constant of the dielectric layer changes. As a result, a high frequency signal affected by the change in relative permittivity is output from the output terminal.

[3. Manufacturing method of dielectric ceramics for tunable devices]
The dielectric ceramic for a tunable device according to the present invention is:
(A) Weigh raw materials (Na source, Nb source, Sr source, Ti source, and Ca source) so as to have predetermined x and y,
(B) mixing the weighed raw materials;
(C) calcining the mixture to cause a solid phase reaction between the raw materials;
(D) After pulverizing the calcined material as necessary, granulate this,
(E) shaping the granulated powder into a predetermined shape;
(F) sintering the compact,
Can be manufactured.
Conditions for each step are not particularly limited, and optimal conditions may be selected according to the values of x and y so that the intended dielectric characteristics can be obtained.

[4. Action of dielectric ceramics for tunable devices]
When the solid solution of SrTiO ₃ and / or CaTiO ₃ is a paraelectric in NaNbO ₃ is anti-ferroelectric dielectric loss (or, Q · f value), the dielectric constant, the dielectric constant for the DC bias field Any one or more of the change rate, the temperature dependency of the relative permittivity, and the frequency dependency of the relative permittivity is improved.
In addition, the dielectric ceramics for tunable devices according to the present invention based on the antiferroelectric material, unlike the dielectric ceramics based on the ferroelectric material, have a low dielectric loss over a wide range of frequencies including the high frequency range. Indicates.
Further, NaNbO ₃ is a hardly sinterable material, but when SrTiO ₃ and / or CaTiO ₃ is dissolved therein, the sintering temperature can be lowered. In addition, it is not necessary to add an environmental load element or an expensive noble metal element in order to obtain the above-described characteristics.

(Example 1, Comparative Examples 1-6)
[1. Preparation of sample]
Na ₂ CO ₃ , K ₂ CO ₃ , Nb ₂ O ₅ , TiO ₂ , SrCO ₃ , CaCO ₃ , and BaCO ₃ (all of which have a purity of 99.9% or more) were used as raw materials.
These raw materials,
(A) (1-x- y) NaNbO 3 -xSrTiO 3 -yCaTiO 3 ( Example 1),
(B) NaNbO ₃ (Comparative Example 1),
(C) SrTiO ₃ (Comparative Example 2),
(D) CaTiO ₃ (Comparative Example 3),
(E) (1-x) Na _0.5 K _0.5 NbO ₃ —xSrTiO ₃ (Comparative Example 4),
(F) (1-y) Na _0.5 K _0.5 NbO ₃ —yCaTiO ₃ (Comparative Example 5), or
(G) Ba _0.5 Sr _0.5 TiO ₃ (Comparative Example 6)
Weighed so that

A raw material powder and a solvent (acetone) weighed in predetermined amounts were put into a polyethylene pot, and ball mill mixing was performed for 20 hours. After drying the mixed powder, this was put in an alumina crucible and calcined at 800 to 1000 ° C. for 5 hours. The calcined powder and the solvent (acetone) were again put into a polyethylene pot and subjected to ball mill mixing for 20 hours. After the calcined powder was dried, it was granulated. Furthermore, the granulated powder was filled in a mold having a diameter of 13 mm, and molded into a cylindrical shape having a thickness of 2 mm or 7 mm at a pressure of 1 t / cm ² (98 MPa).

These molded bodies were sintered at 1100 to 1300 ° C. for 1 hour in an oxygen atmosphere. However, NaNbO ₃ was sintered at 1375 ° C., SrTiO ₃ was sintered at 1500 ° C., and CaTiO ₃ was sintered at 1450 ° C. for 1 hour. In any sample, the rate of temperature increase and the rate of temperature decrease from room temperature to the sintering temperature were each 200 ° C./h.

[2. Test method]
[2.1. Relative permittivity, Q · f value]
Both ends of a cylindrical sintered body obtained from a molded body having a height of 7 mm were polished. Using the obtained cylindrical sample, the relative dielectric constant and Q · f value in microwaves (1 to 4 GHz) were measured based on the double-shorted dielectric resonator method (JIS-R1627). A network analyzer was used for the measurement.

[2.2. Relative permittivity voltage change rate, relative permittivity temperature change rate]
Both ends of a cylindrical sintered body obtained from a molded body having a height of 2 mm were polished. Furthermore, electrodes were formed on both end faces of the cylindrical sample by gold vapor deposition. Using an impedance analyzer, the relative dielectric constant at 100 kHz was measured in the range from room temperature (20 ° C.) to 80 ° C. Further, using a DC voltage / current source, a DC electric field of 10 kV / cm was applied to the surface of the sample, and the relative dielectric constant at 100 kHz was measured at room temperature.
The relative dielectric constant voltage change rate at room temperature and a frequency of 100 kHz was calculated using the obtained relative dielectric constant and equation (2). Moreover, the relative dielectric constant temperature change rate under a frequency of 100 kHz and no bias was calculated using the obtained relative dielectric constant and the equation (4).

[3. result]
[3.1. Dielectric properties in the microwave band]
Table 1 shows the relative dielectric constant and Q · f value in the microwave band (1 GHz). Table 1 shows the following.
(1) When SrTiO ₃ or CaTiO ₃ is dissolved in Na _0.5 K _0.5 NbO ₃ that is a ferroelectric, a high dielectric constant can be obtained. However, the Q · f values are both less than 50 GHz, and the dielectric loss in the microwave band is large. Therefore, Na _0.5 K _0.5 NbO ₃ -based materials are not suitable for high frequency devices.
In contrast, when the NaNbO ₃ is antiferroelectric to solid solution SrTiO ₃ or CaTiO _3, 0.99 or more high dielectric constant can be obtained. Sample No. Except 3, 9, and 10, the Q · f value is 100 GHz or more, and the dielectric loss in the microwave band is low.
(2) When the NaNbO ₃ is a solid solution of SrTiO ₃ or CaTiO ₃ in a predetermined amount, the sintering temperature can be 1300 ° C. or less.

[3.2. Dielectric characteristics in kHz band]
Table 2 shows the relative permittivity, the relative permittivity voltage change rate, and the relative permittivity temperature change rate at 100 kHz. Table 2 also shows the relative dielectric constant and Q · f value in the microwave band (1 GHz). Table 2 shows the following.
(1) When only one of SrTiO _{3 and} CaTiO ₃ is dissolved, when 0 <x ≦ 0.2 or 0 <y ≦ 0.2, the relative permittivity voltage change rate is 0.3 % Or more. In particular, when x = 0 and y = 0.15, a relative dielectric constant voltage change rate of 2.8% was obtained.
(2) Sample No. The Q · f values of 3, 9, and 10 were 50 GHz or less. On the other hand, sample No. ₃ which is NaNbO ₃ —SrTiO ₃ —CaTiO ₃ ternary system. In 24 to 27, a relative dielectric constant voltage change rate of about 1% and a Q · f value of 100 GHz or more were obtained. That is, by adjusting x and y in the range of 0 <x + y ≦ 0.2, a dielectric that has a large dielectric constant change with respect to a DC bias electric field while having a low dielectric loss can be obtained.
(3) The dielectric ceramic according to the present invention has a small difference between the relative dielectric constant at 100 kHz and the relative dielectric constant in the microwave band.
(4) The dielectric ceramic according to the present invention has a smaller absolute value of the relative dielectric constant temperature change rate than Ba _0.5 Sr _0.5 TiO ₃ .

(5) In the ternary phase diagram shown in FIG. 1, attention is paid to the composition in which 0.85NaNbO ₃ -0.15CaTiO ₃ and 0.85NaNbO ₃ -0.15SrTiO ₃ are combined (Sample Nos. ₃ , 10, 25). ~ 27). On this composition line, the tunability increases with an increase in the amount of CaTiO ₃ , and a tunability of 1.0% or more is shown in a composition with CaTiO ₃ of 6 mol% or more.
In addition, even in the composition (sample Nos. 2, 9, and 24) connecting 0.9NaNbO ₃ -0.1CaTiO ₃ and 0.9NaNbO ₃ -0.1SrTiO ₃ , the tunability increases as the amount of CaTiO ₃ increases. It is estimated that a tunability of 1.0% or more can be obtained in a composition where ₃ is 6 mol% or more.
That is, when (x, y) is within the hatched region (including the boundary) in FIG. 1, a tunability of 1.0% or more can be obtained.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The dielectric ceramics for a tunable device according to the present invention can be used for various tunable devices such as a tunable antenna, a multiband pass filter, a phase shifter, and a variable capacitance element.

Claims (6)

(1) A dielectric ceramic for a tunable device having a composition represented by the formula:
(1-x-y) NaNbO 3 -xSrTiO 3 -yCaTiO 3 ··· (1)
However, 0 ≦ x <1, 0 ≦ y <1, 0 <x + y <1. 0 ≦ x ≦ 0.2, 0 ≦ y ≦ 0.2, 0 <x + y ≦ 0.2
The dielectric ceramic for a tunable device according to claim 1.   The x and y are four points (x, y) = (0, 0.15), (0.09, 0.06), (0.04, 0.06), and (0, 0.10). The dielectric ceramic for a tunable device according to claim 1, wherein the dielectric ceramic is in a region surrounded by (including on a boundary line). The tunable device according to any one of claims 1 to 3, wherein an absolute value of a relative dielectric constant electric field change rate represented by the formula (2) is 0.3% or more under conditions of room temperature and a frequency of 100 kHz. Dielectric ceramics.
Relative permittivity electric field change rate (%) = (εr ₀ −εr ₁ ) × 100 / εr ₀ (2)
Where εr ₀ is the relative dielectric constant at a DC bias electric field of ₀ kV / cm,
εr ₁ is a relative dielectric constant at a DC bias electric field of 10 kV / cm.   The dielectric ceramic for a tunable device according to any one of claims 1 to 4, wherein a Q · f value is 100 GHz or more.   The dielectric ceramic for a tunable device according to any one of claims 1 to 5, wherein a sintering temperature is 1300 ° C or lower.

Images