Polymer-type humidity sensor

Details Polymer-type humidity sensor Polymer-type humidity sensor

2001-12-07

2004-10-19

Deb, Anjan (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

Reexamination Certificate

active

06806722

ABSTRACT:

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

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