High-frequency detecting elements and high-frequency heater using the same

Abstract: A high-frequency detecting element that can detect a temperature increase in an instant due to high-frequency wave absorption using semiconducting ceramic with positive resistance-temperature characteristics. The ceramic is composed mainly of barium titanate. A high-frequency heater incorporating the high-frequency detecting element is also disclosed. In the high-frequency detecting element, electrodes are provided on one main surface of the semiconducting ceramic with positive resistance-temperature characteristics and leads (3a and 3b) are soldered to the electrodes (2a and 2b).

Description: BACKGROUND OF THE INVENTION The present invention relates to a high-frequency detecting element and a high-frequency heater using the detecting element. In a heater using high-frequency waves (e.g., microwaves), an object is heated mainly because water in the object absorbs the high-frequency waves. The object, when it contains enough water, is properly heated, but the heating continues even after water in the object has completely evaporated. The methods discussed below are available to prevent the heating from continuing after completion of water evaporation so that the object is heated only to an appropriate temperature. The method disclosed in Japanese Published Unexamined Application No. 62-123226 uses a humidity sensor and an alcohol sensor. The humidity sensor and alcohol sensor are disposed in an exhaust duct and are used to sense water vapor and gas produced from an object in a heating chamber and exhausted from the chamber in order to select a particular cooking method. The method disclosed in Japanese Published Unexamined Application No. 5-172884 uses a microwave sensor and a temperature sensor. A bridge circuit, comprising the microwave sensor and the temperature sensor, is disposed in a heating chamber. The microwave sensor is composed of a thermistor element for sensing the temperature of an electromagnetic-wave absorber as it heats up upon absorbing microwaves, and the temperature sensor is composed of an electromagnetic-wave reflector and a thermistor element. The electromagnetic-wave absorber and the electromagnetic-wave reflector face a heated object and the thermistor elements detect changes in the temperature of the electromagnetic-wave absorber and the electromagnetic-wave reflector during microwave heating. The method disclosed in Japanese Published Unexamined Application No. 60-170188 uses a thermistor. Water vapor and gas produced from a heated object in a heating chamber are detected with an absolute temperature sensor or a gas sensor to control an increase in heating chamber temperature due to a heater, using the thermistor to stop the heating operation. However, all of these methods suffer problems. The method disclosed in Japanese Published Unexamined Application No. 62-123226 has a problem of temperature sensor performance deteriorating due to soiling. The method disclosed in Japanese Published Unexamined Application No. 5-172884 has a problem of the thermistor being slow in responding to an increase in the temperature. The method disclosed in Japanese Published Unexamined Application No. 60-170188 has a problem of a reduction in accuracy due to indirect measurement of the temperature of a heated object. GB-A-2090054 and EP-A-0301652 relate to radiation detectors using a thermistor with a positive temperature coefficient. GB-A-2263173 describes a microwave power detector in an microwave oven. The microwave power detector is formed by a thermistor comprising barium titanate. It is the object of the present invention to provide a high-frequency detecting element having good high frequency detection characteristics. This object is achieved by a high-frequency detecting element according to claim 1. It is a further object of the present invention to provide a high-frequency heater using a high-frequency detection element having good high frequency detection characteristics. This object is achieved by a high-frequency heater according to claim 4. A high-frequency detecting element of the present invention is a semiconducting ceramic with positive resistance-temperature characteristics, and can be composed mainly of barium titanate, which, when the high-frequency detecting element is exposed to high-frequency waves, generates heat and thus heats up the instant it absorbs the waves. A high-frequency heater of the present invention uses a high-frequency detecting element comprising a semiconducting ceramic with positive resistance-temperature characteristics. and which can be composed mainly of barium titanate, the detecting element detecting temperature under the same conditions as an object is heated with high-frequency waves, since the element is placed in a heating chamber containing the object. The present invention will now be described by way of exemplary embodiments with reference to the accompanying drawings in which: FIGs. 1(A) and 1(B) illustrate a high-frequency detecting element of the present invention, 1(A) being a top view of the element, and 1(B) being a side view of the element; and FIG. 2 is a cross-sectional view of a high-frequency heater of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIGs. 1(A) and 1(B) are a top view of a thermistor with positive characteristics and a side view of a high-frequency detecting element. The high-frequency detecting element 4 comprises semiconducting ceramic 1 with positive resistance-temperature characteristics, composed mainly of barium titanate, on one main surface of which ceramic electrodes 2a and 2b, attached with leads 3a and 3b by soldering, are provided. The semiconducting ceramic 1 with positive resistance-temperature characteristics was prepared using the procedure below. Measured is such an amount of barium carbonate, strontium carbonate, lead monoxide, lanthanum oxide, titanium oxide, silicon dioxide, and manganese monoxide that semiconducting ceramic of the composition 0.9798(Ba 0.997-x-y Sr x Pb y La 0.003 ) 1.01 TiO 3 -0.02SiO 2 -0.0002Mn is obtained after a mixture of those substances is calcined. The substances measured are mixed together with pure water and milled with zirconia balls for five hours and incompletely calcined at 1000°C for two hours. The resulting powder is mixed with an organic binder, pressed in dry condition, and calcined at 1400°C for two hours to obtain rectangular semiconducting ceramic of 5 mm x 5 mm x 1 to 5 mm size. Silver electrodes making an ohmic contact are formed on one main surface of the semiconducting ceramic. Leads are soldered to the electrodes to obtain a high-frequency detecting element. The amount of Sr contained in the semiconducting ceramic with positive resistance-temperature characteristics, x, and that of Pb contained in the semiconducting ceramic, y, are varied to change the Curie point in the range from -20 to 200°C. FIG. 2 is a partially cross-sectional view of a high-frequency heater. The high-frequency heater 5 comprises a high-frequency detecting element 4, high-frequency generating means 6, and a heating chamber 7. The high-frequency detecting element 4 is disposed so that a surface on which the electrodes 2a and 2b are not formed faces inside the heating chamber 7 containing an object to be heated. When high-frequency waves are produced with the high-frequency generating means 6 to heat the object in the heating chamber 7, water vapor from the object changes high-frequency output received by the high-frequency detecting element 4. Due to the change in received high-frequency waves, the resistance of the detecting element changes. The rate of change in the resistance of the high-frequency detecting element 4, the heating condition of the object can be known. To evaluate the high-frequency detecting element 4, the rate of change in the resistance of the high-frequency detecting element 4 was measured when the output of high-frequency waves was changed by changing the amount of water 9 in a container 8 placed in the heating chamber 7 of the high-frequency heater 5 with the high-frequency generating means 6 producing 2450-MHz high-frequency waves. A time constant was obtained by measuring the time required for the resistance to be increased by 20% from the initial value after application of high-frequency waves to 50 ml of water in the container. The results are shown in Table 1. Sample No. Curie point (°C) Element thickness(mm) Percentage/100 of resistance change one minute after high-frequency wave output started Time constant (sec.) 50 ml of water 100 ml of water 200 ml of water 1 20 1.0 3.8 2.7 1.9 1.5 2 10 1.0 3.5 2.8 1.9 1.5 3 40 1.0 3.3 2.8 1.9 1.7 4 70 1.0 3.1 2.6 1.7 2.0 5 100 1.0 3.0 2.5 1.6 3.2 6 150 1.0 2.5 1.9 1.4 5.0 7 180 1.0 1.8 1.5 1.2 13 8 200 1.0 1.4 1.2 1.0 25 9 40 2.0 3.3 2.7 2.2 2.1 10 40 3.0 3.1 2.5 1.9 3.5 11 40 4.0 2.9 2.2 1.7 6.5 12 40 5.0 2.5 1.9 1.5 11 If the ceramic thickness exceeds 4 mm or the Curie point exceeds 150°C, the time constant becomes one order of magnitude larger. When, for comparison purposes, a ferrite wave reflector with which a thermistor having negative characteristics was brought into contact according to Published Unexamined Application No. 5-172884 was tested in the same way as described above, it took about 10 seconds for the initial resistance of the thermistor to change by 20%. An embodiment of the present invention can detect a high-frequency wave as a large resistance change, thus offering satisfactory high-frequency wave detection performance. In particular, a Curie point below 150°C and a ceramic thickness of 4 mm or less cause good high-frequency detection characteristics, i.e., a large resistance change and a small time constant, to be offered. A high-frequency detecting element of the present invention comprises semiconducting ceramic with positive resistance-temperature characteristics, composed mainly of barium titanate. When the high-frequency detecting element is exposed to high-frequency waves, the semiconducting ceramic generates heat and thus heats up the instant it absorbs high-frequency waves, so that high-frequency waves can be detected with great precision and, with the time constant being small. Furthermore, one high-frequency detecting element can both absorb high-frequency waves and detect temperature, exhibiting a sharp response to high-frequency waves and correctly following temperature changes. Since a high-frequency heater of the present invention is positioned in a heating chamber containing a high-frequency detecting element, that is, the high-frequency heater is under the same conditions as an object to be heated, the temperature of the object can directly be measured. Comprising semiconducting ceramic with positive resistance-temperature characteristics, composed mainly of barium titanate, the high-frequency detecting element can both absorb high-frequency waves and detect temperature with a high sensitivity in a short time in response to heat, thus allowing proper temperature control to be exercised and a temperature detecting unit to be reduced in size. It will be appreciated by those of ordinary skill in the art that the present invention can be embodied in other specific forms without departing from the scope thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.

Claims: A high-frequency detecting element (4) comprising semiconducting ceramic (1) with positive resistance-temperature characteristics, wherein said semiconducting ceramic (1) is 4 mm or less in thickness and has a Curie point of 150°C or less. The high-frequency detecting element (4) according to claim 1, wherein said semiconducting ceramic (1) includes barium titanate. The high-frequency detecting element (4) according to claim 1 or 2, further comprising a pair of electrodes (2a, 2b) located on one main surface of said semiconducting ceramic (1). A high-frequency heater (5) comprising a heating chamber (7), a high-frequency generating means (6) for producing high-frequency waves in said heating chamber (7), and a high-frequency detecting element (4) detecting high-frequency waves produced in said heating chamber (7), wherein said high-frequency detecting element (4) comprises a semiconducting ceramic (1) with positive resistance-temperature characteristics having a thickness of 4 mm or less and a Curie point of 150°C or less. A high-frequency heater (5) according to claim 4, wherein said high-frequency detecting element (4) is a semiconducting ceramic (1) with positive resistance-temperature characteristics. The high-frequency heater (5) according to claim 4 or 5, wherein said high-frequency detecting element (4) includes barium titanate. The high-frequency heater (5) according to any of claims 4 to 6, wherein said high-frequency detecting element (4) includes a pair of electrodes (2a, 2b) located on one main surface of semiconducting ceramic (1). The high-frequency heater (5) according to any of claims 4 to 7, wherein the other main surface of said high-frequency detecting element (4) on which no electrodes are formed is disposed to face a heating chamber (7) in which high-frequency waves are produced.

Inventors: NIIMI HIDEAKI, TAKAOKA YUICHI, NIIMI, HIDEAKI, TAKAOKA, YUICHI, Niimi, Hideaki, c/o Murata Manufacturing Co., Ltd, TAKAOKA, YUICHI, C/O MURATA MANUFACTURING CO., LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H05B6
IPC codes: G01K7, H05B6, F24C7, H01C7, G01R29, G01J5