JP2012243841A - Tunable element and tunable antenna - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized and low-voltage tunable device using a hexagonal-system barium titanate bulk crystal dielectric material with high dielectric constant.SOLUTION: The tunable element, changing an electrostatic capacitance by applying a voltage, includes: a top face electrode; a bottom face electrode; and a dielectric substance disposed between the top face electrode and the bottom face electrode. The dielectric substance is comprised of hexagonal-system barium titanate.

Description

  The present invention relates to a tunable element that changes its capacitance by applying a voltage.

  In recent years, tunable devices such as tunable antennas are mounted on electronic devices such as cellular phones, personal computer wireless LANs, and PDAs. In particular, portable devices and the like are required to be tunable antennas that can receive a wide range of frequencies such as terrestrial analog broadcasts and terrestrial digital broadcasts while being required to be small and light. Therefore, the tunable element included in the tunable antenna has high tunability in which the capacitance changes greatly with a small voltage (for example, 3 V) obtained directly from the battery voltage without going through a booster circuit or the like. preferable.

Tunability increases as the electrolytic strength increases. For example, in the case of a conventional tunable element incorporating a material such as CaCu ₃ Ti ₄ O ₁₂ or Sr _1-x Ba _x TiO ₃ as described in Patent Document 1, it is required for a mobile phone or the like. In order to achieve high tunability, an electric field of kV / cm must be applied. However, in order to apply an electric field of kV / cm to bulk ceramics or a single crystal tunable element, a DC power supply of several kV is required. Therefore, the bulk tunable device is reduced in size and weight, and energy is saved. Realization is difficult.

Special table 2003-533019

  An object of the present invention is to provide a small, low-voltage tunable device using a hexagonal barium titanate bulk crystal having a high dielectric constant.

  The present invention is a tunable element that changes its capacitance by applying a voltage, and includes a top electrode, a bottom electrode, and a dielectric disposed between the top electrode and the bottom electrode. The dielectric is a hexagonal barium titanate, and provides a tunable element.

  Further, the present invention provides a tunable antenna that receives a signal in a continuous frequency band, a transmission / reception unit made of metal, the tunable element electrically connected to the first end of the reception element, and a reception A tunable antenna comprising: a power supply portion electrically connected to a second end portion of the element.

  The present invention also provides a step of preparing single crystal hexagonal barium titanate, a step of cutting single crystal hexagonal barium titanate into a plate shape, and a metal thin film on both surfaces of the cut single crystal. There is provided a method for producing a tunable element, comprising: a step of vapor-depositing and a step of adhering a conductive wire to a vapor-deposited metal thin film.

  In the present invention, the step of preparing a single crystal hexagonal barium titanate includes mixing a stoichiometric amount of a precursor of barium titanium oxide with a stoichiometric amount of precursor in water at a hydrostatic pressure. A step of solidifying, a step of sintering the precursor, a step of putting the sintered precursor into a heating furnace and growing a single crystal, a step of annealing the single crystal, A method for producing a tunable element is provided.

  In addition, the present invention provides a method for manufacturing a tunable element, wherein the step of growing the single crystal is based on an FZ method.

  The present invention provides a tunable element having high tunability in a wide frequency range having high dielectric constant, low dielectric loss, and low temperature coefficient.

A hexagonal barium titanate single crystal having a diameter of 6 mm and a length of 35 mm is shown. It is the graph which compared the X-ray-analysis result of the single crystal of FIG. 1, and the standard card of ICSD. h-BaTiO ₃ is a graph showing the temperature dependence of the dielectric constant of the single crystal. At each applied field is a graph showing the relationship between the frequency and dielectric constant of h-BaTiO ₃ single crystal. At each applied field is a graph showing the relationship between the frequency and tunability of h-BaTiO ₃ single crystal. It is the schematic which shows one Embodiment of a tunable antenna.

A method for producing hexagonal barium titanate (hereinafter referred to as h-BaTiO ₃ ) used in the present invention will be described. The following method is one embodiment, and hexagonal barium titanate produced by other methods may be used.

A stoichiometric amount of precursor is mixed well so that each material is evenly distributed. Precursor barium carbonate (BaCO ₃ ) and titanium oxide (TiO ₂ ) are preferably used. The mixed precursor powder is hardened into a desired shape and sintered. For example, it is formed into a rod shape having a diameter of 5 mm and a length of 60 mm under hydrostatic pressure, and is sintered at 1600 K for 12 hours. A single crystal is grown using the sintered precursor. For example, a single crystal is grown by the FZ method (Floating Zone method) in an infrared concentrated heating furnace under conditions of a growth rate of 15 mm / h and a rotation speed of 30 rpm. The growing atmosphere is an important condition for stably growing the single crystal. The Ar atmosphere is the most easy condition for obtaining a single crystal. In particular, the oxygen partial pressure is preferably about 10 ⁻⁵ atm which is the residual oxygen partial pressure in Ar gas. A single crystal having a diameter of 6 mm and a length of 35 mm grown in an Ar atmosphere is shown in FIG. FIG. 2 shows a graph comparing the result of X-ray analysis of this single crystal with the standard card of # 75240, h-BaTiO ₃ of the inorganic crystal structure database (ICSD). In FIG. 2, the peak of the standard card coincides with the peak of the prepared single crystal, and it can be seen that the prepared single crystal is h-BaTiO ₃ . In addition, the above method makes it possible to produce a single crystal having a sufficient size for use in a device (for mass production).

Annealing is performed to control the dielectric properties of the grown h-BaTiO ₃ single crystal. Oxygen absorption by annealing treatment can reduce the amount of oxygen vacancies in the single crystal, and can change temperature dependency such as dielectric constant and dielectric loss. The single crystal grown by the above method is annealed in the air atmosphere in the temperature range of 300 to 800 degrees for 1 to 10 hours, and the temperature dependence of dielectric constant, dielectric loss, and dielectric constant is obtained. Change. For example, a single crystal that has been annealed in an air atmosphere at 680k for 2 hours has a relative dielectric constant at room temperature that is reduced from 100,000 to 20,000, but the dielectric loss is 0.1 or less. Furthermore, the temperature dependence of the giant dielectric constant near room temperature is greatly reduced. FIG. 3 shows the temperature dependence of the dielectric constant of the h-BaTiO ₃ single crystal. The relative change amount Δε / ε of the relative dielectric constant in the temperature range of 240 k to 300 k is about 0.1%. Further, since the relative permittivity and the capacitance are proportional, it can be said that the ratio change amount ΔC / C of the capacitance is about 0.1%. Note that the tunable (variable) element (capacitor) materials (Ba, Ca) TiO ₃ and (Ba, Sr) TiO ₃ , which are conventional techniques, have a relative dielectric constant of about 4000 near room temperature, and have an electrostatic capacitance. The ratio change amount ΔC / C is about 10%. Compared with (Ba, Ca) TiO ₃ and (Ba, Sr) TiO ₃ , the h-BaTiO ₃ single crystal has a higher dielectric constant near room temperature, but has a smaller amount of change in capacitance, and the tuner. It can be understood that the material is excellent as a material for a bull element.

  The annealed single crystal is cut into a shape suitable for the device (plate shape, sheet shape, etc.). For example, it is cut into a plate shape having a diameter of 3 mm and a thickness of 0.5 mm. A metal is vapor-deposited on the cut single crystal to form two electrodes, and a conductive wire is bonded to the electrode. For example, the same thin film having a thickness of 1 μm is vapor-deposited on both surfaces of the single crystal and used as an electrode. Further, a silver wire having a diameter of 0.05 mm is bonded to the copper thin film electrode with a silver paste.

  Using an impedance analyzer, an electric field of up to 0-50 V / cm was applied to the cross section of the tunable element prepared as described above, and the relative dielectric constant was measured as a function of the applied voltage at room temperature. FIG. 4 shows the relative dielectric constant of the applied electric field from 0 to 50 V / cm in the frequency range from 10 kHz to 10 MHz. It was confirmed that the relative dielectric constant was 30000 or more up to 1 MHz.

FIG. 5 shows the tunability calculated from the measurement data of FIG. Here, the tunability is the amount of change in capacitance with respect to the applied voltage, where C ₀ is the capacitance before the voltage is applied, and C _v is the capacitance after the voltage is applied.
[Equation 1]
T (tunability) = ((C ₀ −C _v ) / C ₀ ) × 100 (%)
It is expressed as such. In general, a material having a high dielectric constant is desirable as a material capable of obtaining high tunability. Further, the tunable element is required to have a low dielectric loss, a low temperature coefficient, and a low frequency dependency. It can be seen that when an electric field of 50 V / cm is applied, the frequency increases to 35 with respect to 10 kHz, and increases to 10 with respect to the frequency of 10 MHz.

From the above results, it can be seen that h-BaTiO ₃ has a high dielectric constant, a low dielectric loss, a stable temperature dependence, and a high tunability.

  An embodiment of a tunable antenna using a tunable element obtained by the above method will be described with reference to FIG.

  The tunable antenna 600 mainly includes a transmitting / receiving element 604, a tunable element 608 electrically connected to one end of the transmitting / receiving element 604, and a signal feeding unit 612 electrically connected to the other end of the transmitting / receiving element 604. Consists of These components are usually provided on a printed wiring board.

  In the tunable element 608, a resistor 616 and a capacitor 620 are electrically connected to an end opposite to one end connected to the transmitting / receiving element 604. A frequency control circuit 624 is connected to one end of the resistor 616 opposite to the end connected to the tunable element 608. The frequency control circuit 624 applies a control voltage to the tunable element 608 to control the capacitance of the tunable element 608. By controlling the capacitance, it is possible to control the impedance and set the frequency for transmission and reception. The resistor 616 plays a role of preventing a signal transmitted / received in the transmitting / receiving element 604 from flowing into the frequency control circuit 624. The capacitor 620 is connected to the ground 628 at the end opposite to the end connected to the tunable element 608. The capacitor 620 is used to cut the DC component so that the control voltage is not directly applied to the ground.

  The end of the signal power supply unit 612 opposite to the end connected to the transmitting / receiving element 604 is connected to the ground 636 and the tuner circuit 632. The tuner circuit 632 outputs a desired signal, receives a signal received by the transmission / reception element 604, and processes only the desired signal. Further, a ground plane may be disposed on the back side of the substrate, and the ground 628 and the ground 636 may be electrically connected via the ground plane.

  Since the tunable element 608 has high tunability, even if the maximum value of the control voltage is 3 V like a lithium ion battery, the capacitance can be changed greatly. As a result, the frequency of a signal transmitted and received by the antenna 600 can be greatly changed without using an additional circuit such as a booster circuit. When an antenna including a tunable element is used in a mobile phone or the like, the number of parts is reduced, and the size and weight can be reduced. For example, when an antenna including a conventional tunable element is used, in order to receive a signal of 470 MHz to 770 MHz with a maximum applied voltage of 3 V, it is necessary to reduce the impedance by reducing the transmitting / receiving element. For this reason, the antenna gain is reduced and a sufficient gain cannot be obtained. However, when an antenna including the above tunable element is used, a sufficient antenna gain can be obtained because the capacitance of the tunable element can be changed greatly.

600 Tunable Antenna 604 Transceiver Element 608 Tunable Element 612 Signal Feed Unit 616 Resistor 620 Capacitor 624 Frequency Control Circuit 628 Ground 632 Tuner Circuit 636 Ground

Claims (5)

A tunable element whose capacitance changes by applying a voltage,
A top electrode;
A bottom electrode;
A dielectric disposed between the top electrode and the bottom electrode;
With
The tunable element, wherein the dielectric is hexagonal barium titanate. In a tunable antenna that receives signals in a continuous frequency band,
A transmission / reception unit made of metal;
The tunable element according to claim 1 electrically connected to a first end of the receiving element;
A power feeding unit electrically connected to the second end of the receiving element;
A tunable antenna characterized by comprising: Providing a single crystal hexagonal barium titanate;
Cutting the single crystal hexagonal barium titanate into a plate,
Depositing a metal thin film on both surfaces of the cut single crystal;
Adhering to the metal thin film on which a conductive wire is deposited;
A method of manufacturing a tunable element comprising: The step of preparing the single crystal hexagonal barium titanate comprises:
Mixing a stoichiometric amount of precursor of barium titanium oxide stoichiometry;
Solidifying and forming the precursor in water at hydrostatic pressure;
Sintering the precursor;
Putting the sintered precursor into a heating furnace and growing a single crystal;
Annealing the single crystal;
The method for producing a tunable element according to claim 3, comprising:   The method for producing a tunable element according to claim 4, wherein the step of growing the single crystal is performed by an FZ method.