Abstract:
A polymer-type humidity sensor for use in a microwave oven and which has a polymer structure which includes a rubber and a predetermined amount of carbon, and a pair of electric terminals connected to the polymer structure. The polymer-type humidity sensor of the present invention has a rapid response time, durability, excellent adherence to terminals, low hysteresis, and exhibits stability to exposures of high temperatures and high relative humidity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a humidity sensor, and more particularly, to a polymer-type humidity sensor for applications such as a microwave oven and method of manufacturing thereof. 
     2. Description of the Related Art 
     Sensors provide variety of information to microprocessors, which in turn process the information and provide useful information to the recipient. A wide variety of information is processed by computers and microprocessors and transferred to recipients/users such as humans or machines. However, sensor technology, which aims basically at sensing and detecting basic information used by such computers, is far behind computer or communication technologies because of its higher complexity. As such, sensors have become a main hindrance to functional improvement in various systems. 
     Humidity is a universal parameter of common environments and its control is recognized to be very important in a variety of fields, such as industries related to precision manufacturing, fiber, food, and electronics industries. 
     In a microwave oven, an infrared beam temperature sensor, a gas sensor or a humidity sensor is used to monitor the heating or cooking state of the food being cooked. Infrared beam temperature sensors, although having high accuracy relative to the other sensors, are expensive and may cause errors due to a type or shape of a food container. Gas sensors are less expensive than the infrared beam temperature sensors. However, gas sensors are unable to selectively sense desired gases due to the variety of gases generated according to a type of food kinds or even from a single food type. In contrast, humidity sensors are relatively inexpensive. They are also designed to detect water molecules or moisture, which are generated from all types of food upon heating, and to thus monitor the cooking state of the food. With these advantages, humidity sensors are now the most extensively used sensor in general-purpose microwave ovens. 
     A conventional humidity sensor utilizing a wafer of an MgCr 2 O 4 —TiO 2  spinnel solid-solution was first developed by Nitta et al., (U.S. Pat. No. 4,080,564). Subsequently, a ceramic-type humidity sensor was developed utilizing TiO 2 —V 2 O 5 , MgAl 2 O 4 , ZnCr 2 O—LiZnVO 4 , Al 2 O 3 , etc. Afterwards, humidity sensors using polymers were reported. Recently, active research has been directed to the development of thin film or MOS capacitor humidity sensors taking advantage of CMOS technology. Ceramic or thick film type humidity sensors can be relatively simply fabricated, but they show poor reproducibility and contamination resistance, while thin film or MOS capacitor types are fabricated in complicated processes. 
     Organic polymer materials have been widely used in past decades by virtue of their plasticity, lightness, corrosion resistance, and electrical insulation properties. However, the applicable use of the organic polymer materials was limited due to their inherent properties, such as low hardness, wear resistance and conductivity as compared to inorganic materials. However, recent advances in irradiation of polymers have allowed physical and chemical properties of the polymers to be modified. (Chemical treatment, heating, and irradiation of X-ray, gamma ray, UV light and/or high-energy electron beams are generalized as irradiation.) In industrial and medical fields, such treatments find numerous applications, including polymer modification, surface coating, production of heat-shrinkable tubes, thermal and electrical resistant insulators, development of biomedical materials, etc. 
     Additionally, polymerization techniques have been developed to the extent that polymers can have electrical terminals at their opposite ends, thereby allowing the polymers to act as resistance sensors. The polarization of polymers can be achieved by implanting ions or applying strong external fields at a drying step, which is referred to as ionic modification. This technique is implemented by high-energy irradiation, which requires the application of strong electric fields (voltages). The technology of irradiating with high-energy ion beams can also improve the conductivity of polymers and is developed to the extent of being applied to waveguides in communication fields. 
     FIG. 1 shows the utilization of a conventional humidity sensor  4 , such as a ceramic humidity sensor  4 , in an environment such as a microwave oven system  10 . A magnetron  2  generates high-frequency electromagnetic waves, which are radiated to cook food  3 . The ceramic humidity sensor  4  senses a humidity vapor (not shown) from the food  3  during cooking, and outputs signal to a microcomputer  5 , which controls the magnetron  2 . Generally, the conventional ceramic humidity sensor  4  is made from a semiconductor ceramic based on MgCrO 4 —TiO 2 . 
     FIG. 1A shows the humidity vapor contacting a surface  40  of the ceramic humidity sensor  4  composed of a semiconductor ceramic based on MgCrO 4 —TiO 2 . A sensor resistance is reduced when moisture droplets  41  enter the ceramic humidity sensor  4  through numerous pores  42  present in the surface of the ceramic humidity sensor  4  to alter a resistance. 
     The detection of humidity changes using humidity sensor  4  is based on a change in the electrical resistance or capacitance of moisture-sensitive materials used in the humidity sensor  4 , thus change depends on moisture absorption into or condensing on the moisture-sensitive materials. Moisture-sensitive materials for humidity sensor  4  include electrolytes, such as LiCl, metallic materials such as Se and Ge, sintered metal oxide such as MgCr 2 O 4 , ZnCr 2 O 4 , TiO 2 , and SnO 2 , porous metal oxide films such as Al 2 O 3 , electro-conductive particle-dispersed polymeric materials such as nylon, and organic or inorganic polymeric electrolyte films. 
     Humidity sensor  4  made of ceramic materials can cover a wide humidity range and are excellent in thermal resistance. However, the humidity sensors undergo time-dependent changes even when being allowed to stand at room temperature because of the characteristic instability of the metal oxides used. Specifically, the sensitivity to moisture deteriorates in a relatively short time by the hydroxides formed due to the absorption of water onto the metal oxide, or by the deposits leading to a reduction in the moisture-sensitive surface area. For this reason, the humidity sensor  4  is required to be periodically heated to 400-450° C. every 20-40 minutes to recover their performance. 
     In addition, because the moisture sensing capacity of ceramic-based humidity sensor  4  is fundamentally based on the physical absorption of moisture into the ceramic through the pores  42 , it is difficult to reduce the detection error between sensitive devices. It is also difficult to obtain reliable detection properties through the modification of the materials properties as well as the microscopic structures such as pore size, pore distribution, and porosity. 
     Represented by synthetic resins, nylon, etc., polymers are substances made of giant molecules formed by the union of simple molecules, called monomers. Polymer type humidity sensors are designed to quantify the change in sensor resistance or capacitance to determine the humidity. Examples of the organic polymers used in humidity sensor  4  include polyphenylacetylene, cellulose acetate, cellulose acetate butyrate, poly(4-vinylpyridine), and various copolymers. However, conventional polymeric materials used by the current humidity sensor  4  have slow response speed, large hysteresis, and short lifespan. These drawbacks are particularly aggravated upon exposure to high temperature and high relative humidity. 
     Unlike ceramic-based humidity sensors, humidity sensor  4  based on thin film materials, such as polymeric electrolyte membranes, utilize the properties such as hydroscopicity and ion conductivity that the moisture-sensitive materials themselves have. Therefore, the sensing characteristics are determined by the physicochemical properties of the materials rather than by the microscopic structure of the materials. 
     The moisture absorbed into the polymeric electrolyte membrane helps dissolve ionic substance to increase the quantity of operable ions, resulting in a great decrease in specific resistivity. A humidity sensor  4  using the polymeric electrolyte membrane utilizes the phenomenon that the quantity of the absorbed moisture is reversibly changed depending on the moisture content (humidity) of the atmosphere, resulting in a change in the ion production and therefore, electrical conductivity. 
     Thin film materials have reproducible sensitivity because the manufacturing conditions have little effect on the thin film materials. It is also relatively easy to make the physical properties constant from one device to another because a plurality of the humidity sensors  4  can be fabricated on the same substrate. Moreover, unlike ceramic materials, the thin film materials do not require additional high temperature for the fabrication of the humidity sensor  4 . Therefore, special materials or techniques are not required for processes such as electrode formation or lead fixation. These advantages are very helpful for making the device low-priced, small and light, as well as for integrating the device into environmental circuits. However, conventional thin film type humidity sensors require complicated manufacturing and fabrication processes. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a polymer-type humidity sensor with a rapid response time, durability, excellent adherence to terminals, low hysteresis, and low in cost, and a method of manufacturing the polymer-type humidity sensor. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     To achieve the above and other objects of the present invention, a polymer-type humidity sensor according to an embodiment of the present invention includes a polymer structure, which comprises a rubber and carbon and a pair of electric terminals formed to the polymer structure. 
     According to an aspect of the invention, the rubber is a rubber such as NBR-Acrylonitrile Butadiene Rubber. 
     According to another aspect of the invention, the carbon added to the polymer is in a range of 15-20%±5% by volume of the polymer structure. 
     According to a further aspect of the invention, the polymer-type humidity sensor has resistance in a range of range of 500 kΩ-2 MΩ. 
     According to yet another aspect of the invention, the polymer-type humidity sensor has an impedance of 2×10 6  Ω and 5×10 5  Ω at a relative humidity range of 0% and 100% and undergoes impedance change over the whole relative humidity range. 
     According to another embodiment of the present invention, there is provided a method of manufacturing a polymer-type humidity sensor including obtaining a resultant composition which includes a rubber and carbon, and forming a polymer structure of a predetermined shape with a pair of electric terminals formed at the polymer structure using the resultant composition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a diagram of a microwave oven system using a conventional ceramic humidity sensor; 
     FIG. 1A is a cross-sectional view of a surface of the conventional ceramic humidity sensor showing moisture droplets entering the ceramic humidity sensor through pores; 
     FIG. 2 is a perspective view showing an embodiment of a polymer-type humidity sensor according to an embodiment of the present invention; 
     FIG. 3 is a perspective view showing a polymer-type humidity sensor according to another embodiment of the present invention; 
     FIG. 4 is a perspective view showing a polymer-type humidity sensor according to yet another embodiment of the present invention; and 
     FIG. 5 is a diagram of a microwave oven system using the polymer-type humidity sensor according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
     FIG. 2 shows a diagram of a polymer-type humidity sensor  200  according to an embodiment of the present invention. The polymer-type humidity sensor  200  has electrical terminals  220  located to a predetermined area of the polymer  210 . The electrical terminals  220  are spaced apart from each other and are buried within the polymer  210 . A surface of the polymer  210 , such as a moisture contacting face  230 , provides a sensing area for moisture contacting the surface. The polymer  210  and the electrical electrodes  220  of the polymer-type humidity sensor  200  can have a variety of predetermined shapes and placement according to the need of the applicable system. 
     FIG. 3 shows a diagram of another polymer-type humidity sensor  300  according to another embodiment of the present invention. The polymer-type humidity sensor  300  has external electrical terminals  320  of a predetermined shape formed on corresponding brackets  315  disposed at opposite ends of the polymer  310 . A surface of the polymer  310 , such as a moisture contacting face  330 , provides a sensing area for moisture contacting the surface. It is understood that the external electrical terminal  320  and the bracket  315  can be a single piece as shown or can be formed separately by attaching the external electrical terminal  320  to the bracket  315 . 
     FIG. 4 shows a diagram of yet another polymer-type humidity sensor  400  according to another embodiment of the present invention. A polymer  410  of the polymer-type humidity sensor  400  has a cylindrical shape with electrical terminals  420  formed to opposite ends of the polymer  410 . A cylindrical contacting surface of the polymer  410  provides a moisture contacting face  430  for sensing moisture contacting the surface. 
     FIGS. 2 and 5 show a diagram of a typical microwave oven system  500  using the polymer-type humidity sensor  200  according to present invention. The microwave oven system  500  comprises a food cavity  510 , a magnetron  520  and a cooling fan  530 . FIG. 5 further shows that moisture  540  generated from food  550  is directed by the cooling fan  530  towards a sensor location  560  of the polymer-type humidity sensor  200 . Upon heating of the food  550 , the polymer-type humidity sensor  200  located in the sensor location  560  detects moisture  540 . A microcomputer (not shown) connected to the polymer-type humidity sensor  200  detects the change in resistance of the polymer-type humidity sensor  200 , calculates the remaining heating time of the food  550 , and automatically stops the operation of the microwave oven system  500 . However, it is understood that the sensor location  560  need not be opposite the cooling fan  530  but can be located in other areas, such as adjacent the cooling fan  530  or on other walls through which portions of the moisture  540  is exhausted from the food cavity  510 . 
     In one embodiment, a polymer-type humidity sensor according to present invention has impedances of about 2×10 6  Ω and 5×10 5  Ω at 0% and 100% relative humidity (RH), respectively and undergoes impedance changes over the whole humidity range. Also, the difference between moisture sensitivities during a moisture absorbing process (0% RH→100% RH) and a moisture desorbing process (100% RH→0% RH) is as small as 2% RH or less. 
     Using the polarization of ions (H + ), the polymer-type humidity sensor responds to moisture changes over the whole humidity range and represents the response as a resistance change. In addition, the polymer-type humidity sensor is easily fabricated and minimized in its size, so that it is advantageous in terms of production at low cost and integration into appliances. 
     A polymer of the present invention used in the polymer-type humidity sensor is fabricated by utilizing the cross-linking reaction of hydrophilic polymeric materials with hydrophobic polymeric materials. More specifically, the polymer of the present invention is prepared from a rubber or a natural rubber compound that is improved in electrical conductivity. The specific formulation is characterized by the need of the application, such as one suitable for use in microwave oven with superior linearity of sensing properties. For example, a polymer-type humidity sensor device  200  according to the present invention may have a resistance range from 500 kΩ to 2 MΩ, resulting from the composition comprising a rubber (NBR-Acrylonitrile Butadiene Rubber) and carbon. For a specific use such as a microwave oven; a polymer-type humidity sensor  200  is characterized in that the composition comprises carbon in an amount of 15-20%±5% by volume of the polymer structure. 
     By using rubber, adherence between contact terminals is improved, oxidation of the terminals prevented, and an improvement is brought about in long-term stability and durability of a humidity sensor. In addition, it is possible to remove the dew point. For example, by using a natural rubber, dew on a surface of the natural rubber is absorbed by the porous natural rubber. It is also possible to produce the humidity sensor with reliability, at a small size, and at low cost. 
     By using carbon, as a conductive additive it is possible to stabilize the humidity detection properties under conditions of changing humidity and temperature. Furthermore, carbon increases the conductivity of the rubber and is easy to use to set impedance, current and voltage values for the interchangeability needed for the mechanical and electronical junctions. Also, it is easy to set material resistance according to designed geometrical figures. That is, it is easy to change the impedance of the material. 
     In addition, unlike polymeric materials used in conventional humidity sensor  4 , a conductive polymer of the present invention provides a polymer-type humidity sensor  200  with fast response speed, low hysteresis, and a longer lifespan. Furthermore, unlike the conventional polymeric materials, the conductive polymer of the present invention is stable upon exposure to high temperature and high relative humidity. 
     Fabricated by the technology of dispersing electro-conductive particles over a polymeric material, the polymer-type humidity sensor of the present invention adopts carbon to natural rubber to maintain the resistance constant, thereby changing resistance with ease according to the geometrical figures. 
     Because the moisture sensing principle of the polymer-type humidity sensor of the present invention depends on the physical adsorption of moisture molecules to the sensor membrane via pores present in the surface of the membrane, the properties of the humidity sensor can be determined by the properties of the material itself as well as by the microscopic structures such as pore sizes, pore distribution, and porosity. Therefore, the polymer-type humidity sensor of the present invention produces errors within an allowable range and shows reliable operational characteristics. 
     Although a few embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.